You're right here but most of the people who are hunting bounties or in the signature campaign, don't bother to see what they are promoting. Their only interest is to get accepted in the campaign and complete the post count to get paid. None of them wants to promote scam but they only want to earn money.
Yes, once they know this is scam site, they should stop prompting it and remove the signatures ASAP.
It's not always that they unknowingly promote scam projects, but sometimes they knowingly promote a scam project to get a big amount of payment. If you search BitcoIntalk, you will find many projects that have already been promoted by many members.
In order for bounty hunters to earn, they want to join all bounties without knowing the details of those projects. I think Bounty Hunters are responsible for promoting the scam project.