Why, oh Why?

Why does everyone in the NBA insist upon trying to tag guys below the beltline? It makes no sense...

http://www.youtube.com/watch?v=4veKEhpjb1c

I mean, aren't these guys supposed to be like "street". I don't think it's very "street" to tag a guy...I thought you were supposed to face the man to man. Where's Kobe?