In a free market you rarely have to justify rent seeking. If Bethesda are 'rational' then they estimated how much they'd make at various royalties and decided

In a free market you rarely have to justify rent seeking. If Bethesda are 'rational' then they estimated how much they'd make at various royalties and decided that they make the most when they take ~45%, so 'deserves' never entered into their decision and it's reasonable to assume that it never entered into Valve's decision to take 30% either.