Region-lock applies if you are doing the backward compatibility thing. Eg: using it to play PS2 games.

The question of whether they sell R1, R2 or R3 games depends on the demand and how fast the game is available. For example, they prefer to stock up R3 games coz they can get their games quicker from Singapore during release date. For R1, the stock is released in US. So, by the time they managed to ship em' here, everyone else has already purchased R3. So, they might as well stock up more R3. Remember, not everyone has the patience to wait for R1 to arrive. Those not expecting any DLC might as well just get R3.