Yes, Twitter wants you fact check for free. They also want you to write Tweets for free. In return, the hypothetical value proposition is that the tweets you and others read are more likely to be factual. If this is successful at scale, it means that the voters in the hopefully democratic place you live will be more informed and more likely to vote for reality based policies.

So, sure, they don't give you a nickel when you fact check. But that doesn't mean it doesn't (possibly) provide sufficient value to you to be worth doing.