What the f*ck?

I have a question.

Why the f*ck do we cover up or remove the vowels from swear words?  Usually an asterisk ( * ) is used; other times it's an underscore ( _ ) or a simple dash ( - ).  Some people like to get cutesy and use symbols (@$$). 

What I want to know is, when did this tradition start?  And why is it always the vowel?  It doesn't make a lick of sense!  Is f*ck really more appropriate than *uck, fu*k or fuc*?  And what about the word bastard?  (I'm sorry, b*stard.)  Why isn't it b*st*rd?  Is it just the first vowel, is there some kind of exception to the rule that I don't know about?

And what about people that remove every letter except the first and last, as in b---h or s--t?  What about a--hole?  How does that work?  We need some consistency, folks!  We need some rules set in stone!  Where are the Grammar Police when you need them???