Because S[OFU] are Q&A sites. Meta is actually a replacement for the UserVoice bug report/feature request sitea replacement for the UserVoice bug report/feature request site (with a bit of discussion thrown in...). The rules are different because Meta is... kinda misusing the SO engine.
And it's ok because we really do need a bug report / feature request / meta discussion site.
BTW: to the best of my knowledge, no one officially "changed" the meaning of a down-vote. Everyone has their own reasons for voting, it's a private matter, and you might well never know why you received an up- or down- vote unless they felt like telling you. The fact is, posts are allowed on Meta that normally aren't allowed on any of the other sites, and as a result you see voting behaviors that are rare elsewhere (but not unknown: check out votes on some of the more subjective posts on SO...)