Because mercenary motivation is in and of itself a choice. If everyone was motivated by money alone there wouldn’t be non-profits. There is clearly a market for occupations with different incentives.
I think the thing that really upsets people is that the company changed course. People who go to work at a HFT firm are probably all motivated by compensation - and would expect that, because it’s baked into the company culture and business model.
People choose to work somewhere based the reward it offers. If that changes, without their consent, it breaks an unwritten contract.
(None of this matters to the larger discussion but wanted to offer you a perspective of someone who has actually made the choice you are using as an example)