It's not true that they got NO residuals, just that they only got them for a small number of reruns after which there were no more.
Standard TV contracts were payments for one play and two repeats. The casts of "Batman" and "Gilligan's Island" laughed hysterically at the thought that that their shows would actually go into a
second repeat!
William Shatner fought to get chunky pensions for the TOS cast as part of their salary package for ST V. Cynics would say he was worried the cast might have refused to work with him as director, and this was a tempter. De's payment on ST V guaranteed he'd never have to work again, unless he wanted to.