Is it considered poor etiquette to point out when someone else is using poor etiquette?

In some cases, it can be just as rude, if not ruder to point out that someone else isn't behaving properly. But in this case, you have every right to ask people to keep off your yard, and they should have the common courtesy to respect your wishes. Your best bet would be to put up a fence or a hedge to make the shortcut less convenient, if that's not possible, put up a "no tresping" sign.