My registrar has always charged a fee for privacy option, so it's not new for me. I always chose the option as it helps combat scrapers looking for e-mail contacts and the cost for privacy was always going to be less than the cost of fighting the junk.
Same principle as making people pay for unlisted phone numbers,* right? The option doesn't have to cost the provider more money-- may even cost them less-- it only has to be something people are willing to pay extra for.
* Assuming for the sake of discussion that you're old enough to remember when "the phone company" published "the phone book", and "information"/"directory assistance" got you a human.