If another community is mis-filing tags, your community can apply moderation against their community such that your community does not use their tags.
The other way to do this is to have just as good support for +tags in queries as -tags in queries. When adding a tag for no reason is making it less visible to the people who are filtering out that tag, they stop doing it.
Then who prevents totally-unrelated trash from being deliberately misfiled under popular tags?
Subreddits are a way to farm out moderation without paying employees to do it.