Because you clicked the wrong tag in the cloud originally? Because that person no longer belongs under "friends"? Because when you tagged it originally you didn't feel like creating a more meaningful and less generic tag and now it fits better under something else? Or how about just because?

It is something your average user just expects to be able to do. Now I can see it not being a design priority but it really shouldn't be that hard. I assume you can add tags later right? Should just be able to reclick a selected tag and it is untagged.