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.