Maybe I'm slow, but I just noticed that bounties can be offered on meta.SO! So, what other differences exist between meta.SO and other meta sites such as meta.SU?
Meta Stack Overflow is the Meta site for the Stack Exchange network as a whole. Your reputation on MSO is independent from any other SE site, and can be earned just as you would on the other non-Meta sites. As a result, bounties are allowed.
Meta sites specific to a particular main Stack Exchange site, such as Super User Meta, are tied to the main site. Your reputation on a site-specific Meta is refreshed from the main site every hour, with the same privileges as on the main site. Bounties, which are normally deducted from your reputation, cannot be started on a site-specific Meta, because your reputation must remain in sync with the main site.
Well, Meta.stackoverflow really us meta.stackexchange (go there! meta.stackexchange.com). For historical reasons, meta questions for the whole site were shunted there, and splitting off a SO specific MSO from the site proved difficult. It may happen eventually though. MSO/MSE is pretty much a standalone site for asking network related questions.
I'd also note they have a different community culture and attitude towards reputation - bounties are often used to draw attention to a question (even if occationally there's no answer to them). There's also some wierdness (like confused folk posting SO questions on MSO), and such.
The simple answer, really is MSO is a wierd beast, and its a SE site about SE as a whole, rather than a support site as the metas are. It just works differently