Because that is the way the system has been setup.  Bad buyers get a strike if you open an item not paid case,  2 strikes and they are out.  Well not really out, but if you have your buyer restrictions setup they cant buy from you.

Yes strikes are invisible,  but in reality, as you don't know when a buyer is going to buy from you negative feedback on non payment wont really help (except in make an offer I guess)..