- Robinhood was one of many brokerages to implement trading restrictions. e.g. Bank of America/Merrill Edge also shut down buy orders for GME. Citadel does not have the same relationship with these other brokerages. So, to say Robinhood made their decision based on a conflict of interest doesn't hold water.
- Just because Citadel accounts for $xx million of Robinhood revenue, doesn't mean Robinhood stands to lose $xx million if they piss off Citadel. There are plenty of other market makers willing to fulfill Robinhood's orders. Presumably these other market makers would pay slightly less than Citadel, but Robinhood would only stand to lose some small percentage of $xx million.
Putting yourself in Robinhood's shoes, it makes little financial sense to comply with Citadel's demands even if Citadel attempted to coerce them into implementing these trade restrictions.