Spammers exploit anti-spam trap

I don't think anybody seriously expected SPF et al to help with spam -- it doesn't check intentions, it just checks that you have control of the domain you're using in the envelope sender (not necessarily even the From: field in the mail itself).

What SPF does help with is domain forgeries, ie joe jobs and viruses. It can cut down a lot on those pointless bounces you get after someone else has been sending stuff with your email address on it.