I’m not in the USA so I can’t say for sure whether Betsson accepts US affiliates.
I found this on the Littlewoods affiliate program site:
Q. Are you still accepting new affiliate accounts from those in the USA?
A. We are still accepting new affiliates from all over the world. Those from the USA can set up their account as normal and continue to earn revenue on non-US resident players.
However, they don’t offer tiered income so we would derive no benefit from you signing up there. But they have regular monthly tournaments, as well as decent sign-up and monthly bonuses, so by all means go ahead because your players will like them.