Upvoting requires 15 reputation, but the association bonus is taken into account, so if you have at least 200 reputation on any site, you can upvote on all sites in the network.
This is a bit problematic when it comes to per-site metas. The votes there are used to gauge the opinion of the community, and they are often taken into account when implementing new policies.
What can happen is that a post from a per-site meta is linked in an unrelated high-traffic chat room, usually because there is some entertaining drama going on or because someone posted a particularly bad idea. This can lead to an influx of votes from external users that usually wouldn't visit or use the site, but currently still can vote on their meta due to the association bonus.
I think a very low minimum participation on the site should be required to vote on meta issues. This would make the votes on the per-site meta more robust and less easily skewed by being linked from outside.
Not counting the association bonus for the upvote privilege would be enough, then all users would have to earn 14 reputation on the site itself.