Yet the owner of the site is presumably paying for a DB to store all this information. Why?
Usually there is one of two answers:
A) They want to sell you shit, and they need some kind of identifier they can use to unlock content
B) They want to collect info on you to sell to other people
Either way, that's not free. If it's A, I am being asked to invest time in a tool without being told it's potential limitations (there's nothing on the front page mentioning limitations on unpaid accounts), and that's a cost.
If it's B, I'm being data-mined, and companies only pay for data-mining if they think they can turn it into more profit, data-mining costs included. So if I'm being data-mined, it's because someone thinks they can use that data to get me to part with more money at some point down the line than I would choose to part with sans manipulation. That's a cost.
Of course, there's always option C) The site creator hasn't thought about why they need an account system, and made one because that's what all the other cool websites do.
Usually when C comes into play, you can expect account data leakages to follow due to piss poor security practices, and that's a cost.