What else they can they do? They can't devote 70% of their cpu time to processing spam. They also can't just summarily dump emails into /dev/null. I see the "verify" system just the beginning of a "pay me if you want to send me spam" setup. Where spammers will start offering coupons and discounts if you accept their spam. I've even been hearing talk of "email stamps" where each email has to have a email stamp, and you would get a free quota per month. No stamp, no deliver.
Hopefully, one day, agreeing to receive spam will pay my monthly connection costs. :-) I can dream can't I?