1. If you have school in the morning, why are you available for closing time? Because you'd like the extra hrs those shifts usually have and they have extra hrs because of the added responsibilities and situations you have to deal with. 2.if you work two jobs, I will accommodate to your needs as much as possible to help you out but my business is more important than the other guy you are working for. In fact, I do have several people in that situation. There's a guy who I'll never keep him longer in the afternoon so he can make it to his other job, for closing and the he'll be back in the morning. Sometimes he stays longer to close in his other job.
2. 15 min early every shift? Good! Pick up extra shifts when people are sick? Great! Always prepared? Wonderful. You sound like a reliable and consistent employee, most likely you don't care about an extra 20 min just to wait for a customer. That's how I know you have no experience on managing retail, because you learn about people and an employee like that I'm sure he wouldn't mind.
3. If a customer shows up 1min after close we turn them away. 1min before closing time our doors are open and we are ready to serve. Most likely that person isn't doing a week's worth of shopping, they're just picking a few things.

4. What labor law? Idiot. Yes, I'm sure there could be something like that in your state/region, a law that tells people how much they can work and if you do like the extra hrs, sorry can't do it, Go home!

5. Here's my absolutely destroying and final argument, the situation express in this stupid meme, doesn't happen often. It does happen, could be a bit annoying if we get one of those crazy late, shoppers and they don't stay longer than 20 min to half an hr.

Look, none is telling you to forfeit your legal rights, if you are a great employee and do all the things right but also you don't like to be up past ten and wake up before 9 then sure I'll give you hrs, small amount. You get what you put in. Like I said, if something as minor as 20 min after closing (closing store doesn't mean closing shift btw, most places close the doors but they are still working inside) is too much to handle, what am I supposed to do when those shit storm days come around when you might end up working 12, 14 or even 18 hrs and I ask you ( I will ask you), can you stay? You'll turn to me and say, you see I play baseball in the local league and I have the most important game of the entire season coming up, sorry boss. I want those extra hrs when is more convenient to me not when you need to give them to me.