But that's the thing, I was not spamming at all. In fact, the message was quite short.
Well that's just the rule in general. How to enforce it is another thing:
How to interpret the rule is what causes trouble. Basically there will be some GSes who like to keep it up tight and stop caps before they even start becoming a nuisance and warn (proactive) or they are laid back and will wait till it gets unbearable and react trying to shut it down (reactive).
Sometimes depending on who it is or the situation, one approach is better then the other. Now I don't know the circumstances as to why you were warned if it was just you or whatever, I can't say. My suggestion would be to just contact the GS that warned you to get an idea of why they warned you for only using all caps once.
Personally if it's done once in a while and not everyone else follows suit, it's no big deal. Otherwise, would step in if other people tend to be starting to go all caps too or if you kept at continuing with caps.
My 2 cents on the matter.