Few years ago new neighbours moved in, they asked if for a few quid a month they could rent my WiFi. I set up a repeater to forward everything to the café below our flat's WiFi and keep the cash. I don't know what awful shit they'll download.
Fessor is worried about ending up on a list if it neighbours do something illegal. Guest WiFi won't solve that. An always-on VPN in the router might do, but that would cost money, and Fessor is a scam artist who'll risk a ruined reputation for a few quid a month.