If the Bounty Manager knew that the project is scam and still promoted it than they deserve a negative trust.
But if they didn't knew about that, they are no different from bounty participants as they get paid to advertise.
If bounty manager are to be given negative trust for working on a scam project, bounty participants too deserve one as they too promote for payment.
It is a bounty managers job to ensure that the project that he/she agreed on handling is a legit project and that it will not turn out to be a scam. A person shold not be eligible to handle campaigns if he/she does not know how to differenciate a good project and what is not.
I do agree tho it may be best to also tag the participants if a project turned out to be a scam. Maybe that will lessen the account which are solely farmed to get into a bounty without even knowing what project they are advertising.
I have been scammed before by some bounties and it wasn't always the campaign managers fault.