Not everyone (in fact I'd venture most people) don't read the rules sticky (or if they do, they don't remember everything) so they won't know the rule you quoted. However, if the mod closing a thread quoted (or linked to) the violated rule in the closed thread itself it would not be mysterious to the poster why a thread get locked. The OP did not know he was bumping and so didn't know he was in violation. If NUTTSGT or whoever closed the thread had quoted the rule in the thread, the OP would not have had to come here and ask.
The point is to say why the thread was closed in the closed thread so that the poster and others can learn why.
For the record, I didn't delete the thread in question. Those threads get deleted and not closed because closing will still allows them to be viewed. That makes closing them pointless.
"Keeping the thread open" is refering to this thread we are posting in, so that others can learn and see an explaination in this thread. In the end, it's probably pointless as others will not read this thread, the rules in the classified sections or the stickys posted at the top.
The threads that I have deleted in the classifieds, I have sent a PM to the member in question explaining the situation.