Amber holds the purse's strings, and they all - yes, even Ricky - are complicit in allowing that to happen. Amber is very good at manipulation, she's been doing it her whole life: she's manipulated her ex-girlfriends and parents, she managed to take money from girls online while having a girlfriend, she had her foster parents wrapped around her little finger, and it wouldn't surprise me if she also manipulated the social workers who had the displeasure of working with her.
Becky might be the easiest mark Amber ever came across. Eric isn't that hard to push around either, and whatever Eric does, Ricky will accept.
Becky wanted a sugar mama and she got one. IF there was ever any resistance from the boys' part, that was dealt with the same way Amber deal with everything else - she threw money at them and made them shut up. Other than the subtle shade Eric sometimes throws, nothing major will come from any of them as long as Amber is there. They are dumb, but even they know you don't kill the goose who lays the golden eggs.