It's not simply that they don't have any requirement to consider fair use but that there's no incentive for them to do it. If they defend users content that may

It's not simply that they don't have any requirement to consider fair use but that there's no incentive for them to do it. If they defend users content that may infringe or may be fair use (the distinction is up to a court, mind you) they open themselves up to secondary liability. If you're a service provider you're probably going to err on the side of claimants and never reject a single one.