It kinda depends. If a place puts up a sign saying to keep out of their scrap or garbage bins, then it's theft. If it isn't posted, try your luck, or if it's within the road allowance, it's fair game. I also know that when something is returned defective to a store and is under warranty, quite often the manufacturer will simply issue a D.I.F. (Destroy In Field) order instead of paying to have the item shipped all the way back and repaired. If that is the case the store can be held liable for the value of the item if it is not destroyed. It sucks because it seems more often than not, returned items are just fine and it was a problem with the customer after all. At least, such was the case when I worked at Staples. Since I was sometimes the one doing the destroying, I managed to score a few computer parts that were perfectly fine. One case that really ticked me off was at one of the local wood heating shops. They had a huge stack of perfectly good (in appearance, anyway) nice wood stoves sitting out beside their scrap bin, so I went in and asked what the deal was, and if they were free for the taking since there was no sign. Apparently they're units that the store has removed when they do energy-efficiency upgrades in houses, which the government gives a grant for. But the grant requires the destruction of the removed unit.
Unfortunately the dealer here tears down every busted saw that people leave there and keep the parts. They have boxes and boxes of parts even for old models, which they generally sell for half the price of new parts, or less depending on condition. I've also had a couple saws pretty much given to me as fixer-uppers, and if work keeps slowing down I'll probably try getting my name out there for doing a bit of saw work over the winter. Or I might talk to the old guy I get parts from to see if he'd like some help - last time I was there he had a week's work backed up on his shop floor.