Sites can set the difficulty level necessary for their application. Some are under continual targeted attack, others are mainly keeping out rogue automated spambots from their comments section.
The user is typically getting a free service, a better site experience due to less bot traffic, or both. I think sharing the value of their work with the website is a fair deal.
As for using blockchain tech for ledger functions, that is all under the hood: websites can cash out to dollars as they prefer.
(disclosure: work on bot detection at hCaptcha.com)