A similar argument was tried with 'we sell all users data because otherwise we would not be able to run the service, thus we have a legitimate interest.' It did not fly there either.
A service does not have a right to exist. The user has a right to privacy. The users right to privacy trumps the services want to exist. Not to mention that, yeah, there are ways to get similar or better blocking for free, if you have some technical chops at least. I wouldn't fault a small blog for using googles captcha (although the need is questionable), but any company with at least a few employees should be able to figure this out at a relatively trivial cost.