It's basically because most people think of criminal acts as things that happen sneakily. The criminal would be looking around furtively, at night, as he slides up to the bike. Whereas the guy breaking the chain in broad daylight must have lost his key. Especially when he used the angle grinder, no criminal would go to that much trouble (notice he wore proper safety equipment kids!).

That and the general assumption is that most other people are honest. You'd rather assume the guy is the owner who lost his key than a criminal stealing the bike. And no matter what, especially in a big city, you're not likely to be thanked for intervening. If it really is a criminal, you might be endangering yourself by stopping him. And if it's the owner, he's probably pissed as hell that he lost the key and has spent the last 5 minutes with a hacksaw working on his own chain... so when you say hey you stop stealing that bike he's liable to come after YOU with the hacksaw.