Those apps just shouldn't exist.

I don't know how "join your users to a botnet" became some kind of legitimized monetization scheme. Ads are bad enough. What's next? "Participate in a DDOS in order to use our app?"

Add to that list respecting the TOS of the user's ISP so that the user does not get banned, and providing some sort of remuneration if illicit activity through the proxy causes problems for the primary user, and then _maybe_ you could call this all not shady as fuck.

Grey market residential proxy service providers are one of the most common methods of implementing bot spam, social media manipulation and plenty of straight-out fraud.

There's all kinds of things that malicious actors want to do where they value coming from an ordinary (comcast, charter, centurylink, shaw cable, whatever) residential IP.