They don't have to provide the service. They can charge money for the service (but then they probably will go bankrupt because few people are willing to pay for it, because the service isn't that great).
They can show ads on the service. They can even ask people to opt in to targeted advertising.
What they can't do is harvest people's personal data to monetize the service.
In other words: You can give away potatoes for free. You can sell potatoes for money. You can sell potatoes for onions. You can, in many countries, even sell potatoes for sexual favors, although there will be some rules to protect against exploitation. What you cannot do, in most countries, is sell potatoes for kidneys, and you most definitely can't sell potatoes while surreptitiously removing customer's kidneys and hoping they don't notice or care enough because they have two and you're only taking one.
