Read the EULA, they sell you a license to use a product through a service company (Steam). They can revoke it anytime if they get evidence you're not complying with the rules they set (accordingly that those rules aren't illegal themselves).
As it as been said before, Relic hasn't a history of blindness banning players so either Rosbone is playing fool by not acknowledging his own behavior either they may have various reasons and he's not sure which one of them triggered the ban.
In any cases, with what he wrote around this forum, that's probably enough to trigger an end of the license contract, something probably related to defamation of Relic/Relic's employees that you, Rosbone, we, aren't enough versed into law to judge if valid or not but that their legal service/legal adviser most likely is.
As per communicating of the reasons, Relic is fully entitled to not produce their evidences at will and only reserve them in case of legal action. AKA if Relic does so, and so the majority of major gaming companies, when banning players that's because legally it is way better for them to manage it that way.
The appealing process is most likely an internal process and the case transferred to a 3rd party group/manager who'll review it from a neutral point of view and asses if the sanction is valid. It doesn't involve the player because what they asses is their decision making based on the information they had the moment it was taken. They do not seek for more information or to give more information. If the reviewer board valid the sanction, then the sanction is confirmed, if the board invalid it then the sanction is revoked.
I'm not saying it 100% works like that at Relic, but this is the general process of appeal of sanction. If you want to know the reason behind the decision, then you must take legal action against them.
I know that today you only buy "licenses" instead of an actual copy. I'd be interested if that would actually stand up in court case, e.g. if Steam disappeared or something, since this system effectively works exactly the same way as buying a hard copy and everything in the store also suggests that you "buy the game", not a license to play. But that's another topic, so I'll leave it at that.
I don't care what Relic writes in its Eula. Relic being able to revoke the product I bought at any time without compensation or getting any information how I can dispute that, is putting me into a bad spot from the start. It's a system in which they have all the leverage and I don't. I can't just withdraw my money from their account just because they broke their game with a patch or something else that can go wrong. It's a disadvantage of their product and should be counted as their product being lower value. Most people don't care though until they get banned. The majority of bans are rightful (at least I guess/hope so), but there definitely are cases of wrong banning. The better they do their job, the less wrongful banning cases there are, but they will still be there. Relic (or any company for that matter) not providing any realistic course to appeal for a ban further lowers their product value.
The current practice of not giving any information might be industry standard, but is inherently anti consumer. I don't see why I should not criticise that system.
I don't know what factors exactly lead to Rosbone's ban. I guess him joking about the appearance of Relic employees was the trigger, maybe there's more, maybe not. I have literally no idea, I have never talked to any Relic employee ever. Which also leads to the next question: Should Relic even moderate based on behaviour on other platforms? And which platforms? And in which cases?