I'm sure Prowse got paid for his work on the OT. It'd be nice if everyone in the movies we like were filthy rich, but I don't see any reason Lucas should be more generous than he legally needs to be with anyone who has a contract. Why should he be judged more harshly than the rest of Hollywood?
It's not like no other 6'6" actors could have pulled off Prowse's brilliant and nuanced performance.