How are random people displaying advertisements of mixers in exchange for some Bitcoins = scammers?
Because a "mixing site" is just someone else's Bitcoin wallet that you can use to scam people with, by using someone else's wallet to collect the scam money, the scammers can keep repeating the process without ever getting banned:
It doesn't matter if they "couldn't have known" that Chipmixer would scam everyone, it matters that they "could've guessed" it would. After the Chipmixer scam, its promoters immediately switched to promoting the next scamming site. And after that mixing site scam predictably took everyone's money, guess what? These same promoters would move on to post ANOTHER mixing site scam in their signature.
These scammers understand a mixing site is just someone else's Bitcoin wallet. This allows forum users participating in these signature promotions to scam over and over and over again by just blaming their boss for taking all the money (while they get paid their cut of the scam up front for helping spread the scam's deposit link across the forum).