What is reputation on the main site?
You could say it's a representation of your technical skills and knowledge and how well you put it into answers. In that case, the read-only reputation on the corresponding Meta site really is something else, because Meta doesn't care about your technical knowledge. For Meta, it's simply how much you contribute to the main site community that is being discussed on Meta. If you've never been to Meta, but been a contributor on the main site, you have Meta reputation.
But that isn't the whole story. You gain main site reputation from answers, questions, bounties and (a bit) by editing others' posts. It always has been a measure of the amount and quality of your participation, and all of it (except self-inflicted downvote costs) is provided by other users, either by them up and down voting, assigning bounties, or approving your edits.
Obviously, we care not only about users providing raw content, but also about contributing in other ways to the community. This has always existed through badges, but since edits, like fixing ancient posts or contributing tag wiki descriptions are awarded reputation, even indirect contributions have reputation value.
Meta participation is one such indirect contribution. By contributing good content to Meta, be it bug reports, feature requests that are realized, discussing what the community values and respects, and building a knowledge base of articles on the use of the site (support questions), contributions to Meta have value to the community as a whole. And don't think Meta isn't part of the site. Meta is part of the site as a whole -- every user automatically has a corresponding Meta account and can contribute to it.
Now, ignoring the special case of MSO, there is no reputation rewards for contributions to Meta. This can have several reasons. One, of course, is the purity of the reputation value. It should only, or at least, primarily represent the contributions to the main site. Another might be that votes on Meta are easier to get, since the topics are easier, and less strict about what is correct. I am sure there are others.
In this Meta topic, I request we rip this nice separation apart and allow bounties for Meta topics. Here's why it's not a bad idea:
1. They cost reputation
Different from main site votes, they cost. We have decidedly fewer answer downvotes than the free question downvotes on the main site, very likely because users don't like to pay the -1 cost for posts that'll get deleted anyway. On Meta, users are free to express any opinion by up voting and down voting whatever they want, since it's free. Users likewise aren't pushed towards posting Meta topics for their expected popularity to farm reputation.
Bounties cost reputation, even if there's no eligible recipient, or any recipient you like. It's not like we generate reputation from nothing, like with votes and edits. I have to pay X reputation so that another user receives at most X reputation. It will be pretty unlikely for a user to gain enough reputation from Meta bounties to significantly outweigh his regular contributions. This is almost mandated by the cost of bounties and the fact that you need to know how the site works to be able to contribute value on Meta. You just don't know the site if you haven't provided value yourself, and then cannot really contribute to Meta in turn.
2. Reputation comes from main site contributions
You cannot pay for Meta bounties using Meta reputation, because it doesn't exist as an independent entity. Your reputation comes directly from the main site. You need to pay for getting additional exposure by having contributed an equal amount to the main site.
This is a good thing. It makes Meta bounties more valuable, because you only have one reputation budget to spend on bounties instead of two.
3. It doesn't introduce new problems
While we introduce some crossover between good Meta participation and good main site participation, it's not a problem that can somehow lead to exploits. If someone want to transfer some reputation to another user, it only requires a single answer on the main site to do so. We encourage users to provide bounties after answers are posted if they want to express their gratitude.
4. It helps
Let's face it: Meta participation on this site is lacking. You can probably count on two hands the regular users on this site, and could still cut off a few fingers if you were to exclude diamond moderators from this count. This feature would hopefully increase interest in some Meta discussions and therefore the Meta site as a whole.
We already have featured for topics that are advertised on the main site. It just doesn't help a lot (featured for over a month, less than 10 views per day and no real contributions*).
Bounties are a way for admittedly the higher reputation users like me to try to get more views for topics we consider important. And most Meta topics posted by higher rep users (excluding me perhaps this past month) are relevant! Since the entire site profits from Meta contributions, be they community policy decisions, reference support topics, or to get others to consider the merits of a feature request or a bug report, everybody wins. It doesn't "buy upvotes". It simply gets much needed attention.
It also enables out of band reputation transfers for any reason not discouraged by moderators (topics must survive for two days before they're eligible), something that can power special events such as the community wiki sponsorship suggested a while ago and which are pretty much impossible to do without team support otherwise.
While from a certain purist point of view you might be right, not having Meta bounties is counterproductive. Child Meta bounties enable us to do good by hopefully increasing Meta participation and have no downsides that aren't already present through main site bounties.
* In a way, this topic is an effort to add a tool to the box of making Meta more relevant and therefore a partial solution to the Actually changing things topic.