I would wager Flickr stopped growing significantly a while ago, and therefore doesn't benefit from any marketing that people with free accounts might do for them.
And as the saying goes in creative industries: you can't eat "exposure." The only way to make money is the charge money, and it's generally worth it even if it means pissing off people who expect stuff for free.
Heck I bet this change will drive up awareness of Flickr. I can't even remember what I have there, but now I'm going to look up my creds and log in to find out.