Users are allowed to control how they view content on their own computer. Facebook has chosen to give the content out for free and users may choose to render the content however they wish.
And the user is in no way obligated to look at the ads. If I start a business giving away things for free expecting the users to pay me back later but in no way requiring them to and it doesn't work out then thats just a bad business model.
Facebook is free to charge for the service.