Getting the timing right on shop kills makes it very different from the tank. It's very exciting thinking they might be leaving just as you pull up. This has happened a few times and some great gun scraps have ensued. One time a guy was camping on top of ammu-nation waiting for me.
Your intentions are still cheap, whether or not the outcome was fun or not.
Like other's said, you're trying to kill someone who most likely can't even defend themselves. I really don't care if I see someone trying to sticky bomb someone for no reason while they're both on the move in cars. It's fair game.
You see when I check the map before I go in the shop, and between purchases in a shop I AM defending myself. GTA:O is as much about having information (situational awareness) as it is about parity of weaponry. Let me emphasise again, this DOESN'T happen to me because I AM CAREFUL. That IS defence.