Why do so many people put the inside pedal down when cornering?
People put the inside pedal down because it's the natural thing to do. You want the bike to lean to the left, so you press down on the left side; you want the bike to lean right, so you press on the right.

Putting the outside pedal down is a learnt behaviour, so it's only done by people who have been taught it. Also, most people don't cycle fast and aren't on road bikes, so they're not leaning the bike much and they have reasonably high bottom brackets. Pedal strikes simply aren't an issue for most cyclists.