Typically, Stack Exchange sites have the restriction that only users with at least 5 reputation points on the main site can post on meta. In this regard, MathOverflow is different. (It is listed also in the network-wide FAQ among exceptions: Reputation requirements compared, current revision.)
As far as I can tell, this was changed mainly to help during the transitional period when MO was being migrated to Stack Exchange network, when several users might have had various problems. The relevant discussion can be found here: Allow posting on meta without any reputation for the first week. You may notice that the feature request there starts with: "Would it be possible to remove the reputation requirements for posting on meta for the first week, while we iron out all the login problems people are having?" So from the start on this was probably intended just as a temporary thing.
I'd guess that since several years have passed since then, this is probably no longer needed to help users with the transition to the new network. And there are also some disadvantages to lowered reputation requirements for posting on meta.
- Users unfamiliar with this site might post completely off-topic stuff on meta. (5 reputation points are not that much, but at least the user has to do something on the main site and gets at least very basic knowledge of MO. Recently we have seen several rather off-topic posts on meta. To include just two very recent and very off-topic examples, here are two questions that were posted not too long ago: Please steer me toward a legal organisation and I need your help for a new theory . Both of them are now deleted, but users who visit meta probably remember them - IIRC on one of them there was even a comment that the post is a perfect example of an off-topic question.)
- First time posters can easily post on meta instead on main by mistake.
- There is no post ban on meta. So somebody who is banned from asking on main site can simply continue posting on meta. (They do not get any answer and might eventually get suspended, but still something like this might be quite irritating.)
- This is one of the very few meta sites where I saw spam posted - typically users posting spam have accounts created solely for that purpose so they start with reputation 1. And since meta is visited less that the main site, it might take longer until the spam post is flagged and removed. (Well, I only saw three spam posts on this meta in total, but still... it would be better if there were none of them.)
Proposal: I suggest to change this back to 5 reputation points - as on other sites. (Unless the discussion here shows that there are some strong objections against this from the MathOverflow community or from the moderators.)
Personally, I considered this to be change where it is clear-cut that it is better to change this back to the normal state. But since the moderators responded neither to the comments in the old discussion suggesting the change nor to my recent inquiry in chat, perhaps this is not so clear cut as I thought and it should be discussed first before the change is made; if only to find out whether there is some support for this idea. (And, after all, posting on meta is the standard way to ask for some change - definitely more standard than comments or chat messages.)
One of the reasons that I posted this now is that in the following weeks there is probably going to be more communication between MathOverflow moderators and Stack Exchange team than usual - in connection with the changes to the site theme. So if they are already communicating about other things, moderators can also request this change - provided it gets some support from the community.