I believe the "scammer" parts comes from them wanting to leech data off the users of the given extensions. They are upfront about their intentions, I think, so scammer is maybe not the appropriate term. "Scummer" maybe.
I agree with calling them “scummers” but “scammers” is also apt as they intend to scam the user of the extension - not necessarily the developer. Reading those emails brought to mind Bill Hicks' famous rant about “Satan's little helpers”: https://vimeo.com/36651896
It seems like it is a series of emails from one sender. The first email is about sharing users IPs. What can such scammer do with user IP?
They want to leech bandwidth off the userbase which become 'bots', to resell as a distributed proxy/botnet service from my understanding at least.