I wouldnt "Ignore" the signs either, but hey... I dont exactley look for them. Claim ignorance and leave if approached and keep my money.
One thing I consider even though I dont know the legality indifferences, most businesses are in leased units in strip malls or rented buildings around here. Its not their private property they are trying to ban you from. Im a bit more indifferent to their ban in thoes situations.
Home Depot... I know they have seen mine. I reach and strech while Im getting stuff and loading the truck. Hard to hide a hip carry when tossing lumber. I have noticed slight changes in how they act, clam up mostly, but never approached. Very helpfull staff too.