They are following the natural and good inclination to "follow the money" but because they're liberals they think it's like literally shady guys passing manilla envelopes.
It would explain why bribery is so easy. As long as you don't say, "this is a bribe!" and give someone a sack of money with a dollar sign on it (or literal gold bars), then you can get away with it (see the Supreme Court trips and gifts, lobbyists for Congressmen, the whole donation ecosystem, everything Trump has done this term and the last between having people stay at his hotels to his Qatar jet, etc.).
Well at least certain people can get away with it. It seems very inconsistent. Most grunt workers can't accept a movie ticket from a client, let alone bigger things, for example.