Lifted straight from the rules (which you would have noticed if you actually bothered to read the rules like a normal person does before asking for clarification on said rules):
"A sat account must be played for their own benefit. It is not allowed to abuse account sitting. Account sitters that intentionally destroy or seriously damage an account they are sitting will be punished."
I think 'intentionally rendering troops useless and then regularly attacking' falls under the 'seriously damage' catagory.
Rendering troops useless was one situation, attacking was another that would only come into play if the first was allowable. Basically, the logic for why it would be allowed is this. Sending the troops on a several day trip is not technically damaging it. It is simply a long farming run. He is not losing any troops and I am not allowing anyone to attack him during the sitting period. That is why it is not "seriously damaging" the account, as no actual damage is being done. Farming it afterwards is unrelated to the sitting. The argument for that is that even if any crime has been committed during the sitting period, your actions after the 24 hour period is up are not against the rules as they no longer relate to you account sitting him. So even if you broke the rules regarding what you can do as a sitter, you are not breaking any more rules by attacking him after the waiting period.
Not saying that I actually agree with those things, it's just that it seems to me like you can take advantage of sitting someone without technically breaking the rules.