There is a json_metadata field included with comment operations on the blockchain which can contain arbitrary data. Ideally the information about the tribes in which a post can be upvoted would be added as a new property there and not by bogarting the existing tags feature (which is also stored in the json_metadata field btw).

The issue is that if a new, custom property was used, then it would only be possible to publish posts that can accept payouts in SE tokens through UIs that support this new feature. So by using tags it's possible to post through any front-end and have the post be eligible for SE token rewards without those front-ends needing to do anything to support it.

FWIW my original design called for the use of a new json_metadata property and not to use tags, but it was implemented using tags initially just because they were easier to use for testing (before there was nitrous or any of that), and it just stayed that way.

Ultimately though, I think front-ends can come up with a lot better ways to group and organize content than the existing tags feature, so maybe the fact that tribes are messing with the use of the tags will encourage some front-ends to start experimenting with other mechanisms.