I'm a regular "curb shopper" and my personal rules of thumb are these: If I can't reach it with one foot still touching the curb, then it's probably not smart to reach for it at all. If it's not piled in with other garbage, then I probably shouldn't consider it garbage. If it's in a can/bin marked with a trash company's branding, then it's a no-go, no matter what.
Of course, some things are very obviously intended for garbage that may be sitting in a driveway. If I want it badly enough, I'll knock on the door and ask. Aside from propriety, this "opens the door" (sorry for the pun) to asking if there are any other scrap materials. If nobody answers, then I move on empty-handed. Not worth getting busted for anything that could fit in my van.
In your particular situation, it may be a bigger problem that you put stuff into that dumpster. I would suspect that somebody was trying to dump their old TV in there, and it was too heavy so they just left it in the driveway. When you started putting stuff into that dumpster, you put yourself at risk for "dumping" on property that wasn't yours. Of course I'm making some assumptions, but I think that's a pretty realistic scenario.
As others have said: you were in the wrong, here. I suspect that any chance of fallout is pretty slim, though.
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