Well, Renmiri, what you're saying, in essence, is that you could steal a TV from Best Buy and then destroy said TV 24 hours later and you would no longer be breaking the law. You still stole the TV and regardless of whether or not you still have it, you stole it. You can get in trouble with the law even after you destroyed it.
Same with piracy. Just because you deleted your ROM doesn't mean you're in the clear. The fact is that you still downloaded it, so you're still in a position where you could get in legal trouble for it.
Just because you destroy the evidence that you've broken the law, doesn't mean you're in the clear. Chances are, there's still evidence you had that ROM and chances are they could find the pieces to that TV.