I wasn't familiar with the term, so I just searched the phrase "residential proxies", and the space seems even more sketchy than the usual VPN peddlers who promise the world and more. Are they using malware-infected PCs, or what?
Some of them use the endpoints using their app. Which means a lot of people become the spammers just by joining the network. Of course it's mentioned in the middle of their T&C, but who reads that...