The law is a filter because of the obvious chilling effect: do you really want to risk an inevitable leak where your co-workers will potentially look up info about your porn habits tied to your real name?
Why would it have your 'habits' tied to it? It's meant to ensure you're over 18 or whatever age - what possible reason would it have to identify individual URLs a particular identity visited?
Money. Marketing. Targeted advertising. Etc, etc, etc. Pornhub (and others) are completely free and use more bandwidth than I can wrap my head around. How do you think they pay for that stuff?
Pornhub sells a subscription service with additional features / content.
Renting a porn vhs and having the guy photocopy and file your ID is a much bigger deterrant than just having to show your ID to the clerk.