Kroete, as Aristotle put it "ignorantia iuris nocet". Also even if the accused did not know the rule they had broken, Ignorantia juris non excusat. Now I am not saying the accused broke any rule, but defending someone by saying "no one could verify there was a rule broken that has been known to the accused beforehand' only allows people to break the rules and claim ignorance which is purely unacceptable whether it is true or not.
This depends on if the rule is stated in an understandable manner and in a place were everyone has access to.
If the jurisdiction fails to communicate a set of rules, it can not expect anyone to comply.
If the accused person is too lazy to read or too thick to understand a(n otherwise understandable) rule, Aristotle comes into the play.
In my opinion, the clan league rules are not clear enough on this topic. It is obviously very hard to think of all possibilities beforehand, so this drama here should lead to a clarification in the rules (and yes, this practically means the rules will get longer and longer from clan league to clan league).
In the end, it will be up to the clan league manageres to decide if it should have been clear enough that account sitting is forbidden for clan league games or not anyway. But the rules as they are now leave room for interpretation, so they should be changed to clarify things.