Acting as escrow for a signature campaign is the opposite of "aiding and abetting" them. They are unlikely to pay their participants anything if the funds are not escrowed, Quickseller is preventing a number of people from being scammed by doing this.
This is an argument I don't get. The account was already marked by BadBear. If they couldn't get an escrow they wouldn't have been able to get the participants. Quickseller could've given another rating on top of BadBear's and made the campaign dead. Instead he became escrow which allowed them to get users and had the signature displayed across forum. It was lucky that they folded so soon, otherwise this signature campaign would've caused more damage.
Protecting those participants doesn't make sense. They should've been marked for supporting a scam company, like devthedev had done earlier. Getting a reputed escrow added a certain amount of legitimacy to the campaign. To the ordinary participant, since Quickseller was with the campaign, they figured they are not doing anything wrong either.
Furthermore, numerous trustworthy members have escrowed signature campaigns for scams, such as Dooglus and Mitchell with dicebitcoin.
This is not the best example as dicebitcoin didn't look to be scamming initially. A better example is Tomatocage, becoming the escrow for the account trade of KingofSports. In that he was directly helping a scammer earn some money. This is something which is accepted here for some reason and doesn't make sense to me.