I'm not so sure. It may just be me thinking from a moderator (aka it should be fair/fun for everyone) standpoint, but I wouldn't think a warning needs to be given for exiting the game, leaving other Toons to fend against the boss for themselves. I would never think of doing something like that in TTO, it just seems like common sense to me that you wouldn't want to cause harm to those other Toons and help out.

Remember that some Toons don't get the concept of SOS Shopping, or even know it exists -- they just want to play the game, rescue the Toon, and have fun, they may not be in it for the rewards.

Look at it from a perspective of a Toon like that:

They're there to have fun, and assume others are too. They don't get the idea of leaving the boss battle because there's a reward you don't like, they're not even considering that exists. So they're all ready with their team, ready to head into the boss battle. But suddenly, a Toon (maybe even two) leave. They probably won't think anything of it, maybe it's possible they crashed or disconnected by accident. But say they see you again, and you do the same thing. And again, and again. They won't understand, or possibly just think you're trying to cause harm to the group.

People who do understand but want/need help in a boss battle probably get frustrated because no one considers the team... they only consider themselves and the rewards that they're getting. They have no desire to help out the group they're participating in, and that is selfish. That may be putting it bluntly, but it's entirely factual, even if an SOS Shopper isn't meaning to be selfish at all.

Therefore, since you're harming the group by not participating in it, that's considered intentionally harming other Toons (aka greening variant), and against the rules. I can speak with you about ban lengths.