I realize that this is really small potatoes in the grand scheme of life and a trivial thing to be mad about.
I don't think it's trivial at all. It's YOUR lawn, you work at maintaining it. For someone else to dump their garbage on your lawn (which is what this basically amounts to) is disrespectful and really quite rude.
I'd be pissed off too. Like I said, it's a lack of respect...and if they're dumping trees on your lawn while you're there, what on earth would they do if you were gone on vacation?