This is because most tube sites are being run outside of current U.S. law. They're a mix of copyrighted and unverified amateur content, with no attempt whatsoever to comply with 2257 regulations. If you spend any time perusing the adult industry forums, you'll pick up on this pretty quick. Most adult industry professionals are eagerly awaiting the crackdown on tube sites, which simply hasn't happened yet.
You won't find any affiliate programs for tube sites, because they make all their money off of advertising.
I was thinking about starting some adult sites a couple years ago, and did my homework. Anyone wanting to start an adult site in the U.S. should really sit down and read U.S.C 18 - all of it. When I realized that in order to fully comply with the law I'd have to open my place of business (e.g. my home) to random federal inspection, and face a possible 10 years in prison for not having my records properly cross-referenced, I kinda lost all interest.
People get away with referring back to their sponsor's 2257 page (or in the case of most tube sites, omitting it altogether) because there are many millions of adult websites and only a handful of judges and DOJ lawyers. So you can get away with not being compliant if you're willing to risk winning the reverse lottery of a massive fine and long prison sentence.
Me, I'm going to wait until 2257 is amended or repealed - it's facing another challenge this year; it can't last forever, seeing as it's blatantly overbroad and unconstitutional worded the way it is now.