If someone searches Ask Ubuntu Meta and they get posts that were not created by anyone in the Ask Ubuntu community, that would be at least as unintuitive.
If anyone attempts to implement a feature like this, it would also be necessary to consider what, if anything, to do with all the other sites' metas. (Every Stack Exchange QA site has a meta--for example, here's Gardening and Landscaping Meta.) And it would be necessary to separate the results so that searches on smaller metas--like this one--don't have their results drowned out by posts on bigger metas (like Meta.SO itself).
Some of the answers on Meta.SO (there is no "Meta Stack Exchange" so I presume this is what you're talking about) are incorrect for some other sites because there are some differences between sites. For example, some sites support syntax highlighting; some sites support mathematical formulas; and because of the special importance of citations there, Skeptics lets new users post 50 links in a question, while the limit remains 2 for every other site. There are numerous other differences.
Even more importantly, different sites have different communities and thus different community standards. Community consensus on one site will often differ from community consensus on another site. That most people on Meta.SO think something doesn't necessarily mean it's followed elsewhere--that site does not just serve as a "central hub" of meta sites, but also is the specific meta site for Stack Overflow.
If a question has been asked on another meta (even on Meta.SO), but not here on Meta.AU, it's not considered bad to ask it here. The system is deliberately designed so that questions cannot be closed as duplicates across sites, and only when a question is manifestly unsuitable for a particular meta is it migrated to another (this is rare). It's not bad to have similar questions answered in multiple communities; often, the answers can be targeted to the needs or differences of the community.
Since results from one site's meta (even Meta.SO) don't always apply to another site's meta (such as ours), including search results from other metas would probably lead to considerable confusion. Thus, it should be avoided, unless the nature of the results and the metas from which they're taken can be explained. This is somewhat unlikely, as the degree to which information on one meta applies on another is complex and probably cannot be conveyed to someone who has not been to the other meta and looked around. And if they've done that, they know how to search there themselves. (It's also important to take note that any explanatory text that is offered will likely be ignored by a significant percentage of users.)
In conclusion, I'm not entirely opposed to all possible features that would allow one meta site's search box to give quicker access to other sites' metas, and it's true that sometimes Meta.SO is an excellent place to search, even for issues encountered on or ideas devised in relation to a Stack Exchange site other than Stack Overflow itself. But this feature would have to clearly present those results as secondary, and I cannot think of any way to do that without becoming confusing. So if this feature were successfully implemented, it would probably be as an efficiency feature for experienced users, and would be of minimal utility to newcomers.
I will propose an alternative, though: Sites' Meta FAQ's (like ours) could contain a section that briefly explains the relationship between the site's meta and the main meta (Meta.SO), and link to Meta.SO. If that were done, then users would learn about Meta.SO by reading the FAQ on non-SO SE sites.
