Because tags are supposed to be for categorising saves. Those are not categories or objective descriptions, they are opinions which should instead be expressed by means of an upvote (and possibly a comment too, if you want).
@jacksonmj the current banning process is too weak. Good1 and cool2 are allowed. I think that you should filter ALL tags CONTAINING the banned words
http://en.wikipedia.org/wiki/Scunthorpe_problem
Also c00l, coool, kewl, etc.
A tag whitelist might be nice though.
But that would be fairly tedious. Having to add every word applicable in the english language :/
Does that mean if the tag "ban" is banned, the tag "banana" would be banned? (that was just a silly quick example).