I tend to agree with you. But it did bother people whose topics were so tagged, and I used the PG versions.
An overriding ethos of A2K is for the users to be given a great deal of the power to customize their experience here, rather than something more top-down. For example, people are very infrequently banned, but users can be ignored. What one person finds offensive another might find funny. So the user who finds a given poster offensive can ignore him (and not see his posts to be offended by them), while another user who finds that poster merely funny can still see him.
So for example, rather than having moderators delete "silly" tags (who decides? where's the line? who has time to keep an eye on these things?), things are more likely to be tweaked with code and such. It's unlikely that more than one person would tag your thread on Existential Philosophy "Twirlip wears wifebeaters," but a few would probably tag it "Philosophy," "Existential Philosophy," etc. So if the tags that have been applied several times crowd out the tags that have been applied once, then the community can take control.
I suspect (without knowing for sure, I am not speaking authoritatively here, just musing) that something like that is resulting in the "philosophy" tags not sticking, if they were only applied once. The community can take control there by tagging anything that looks like it belongs in the Philosophy forum with "philosophy," to help it not get crowded out by other tags.
I'm really not sure of the latest on the whole tag thing, I know a lot of things are in process and in flux. That goes to the "sticky" idea, too, I don't think things are really finalized enough yet for something like that. Feel free to ask questions though and we'll do our best to answer.