You would be surprised how big of a hammer ISPs will use when they are told to hit something. They live in a very different world than many modern web software companies - they are the plumbers for lots of things you take for granted, and look at the world the way a plumber does. Thanks to TLS, the plumbers can't see the HTTP headers to figure out what's actually flowing, so they sort of end up whacking all of it.

Generally, low-reputation IP addresses are associated with scams, spammers, and other similar things. Gaming somehow gets lumped into this bucket in some jurisdictions, but that hurts you worldwide (similar with other "sin businesses" like porn). These blacklists get published (I think there's some parts of BGP that make this happen, but I'm not quite sure what the mechanism is), and being on any one of them hurts your traffic everywhere because it becomes suspect.

I agree with you that this mix of compliance, engineering, and sales is gross. If this was the issue, they should have just told the OP.