frankly, in any job you will normally stop for minimum a few minutes here and there. bump into a friend and ask how the pub was the other night, this happens at most work places frequently. officially you should simply never stop working at all the whole day, but life isn't like that and i've never seen anyone get in trouble at any places i've worked for such. but occasionally places are more messy, or something took longer than usual and an extra 15 minutes is needed. because its work, it needs to get to a certain point, especially retail type jobs, crappy as they are. if its mess create on your shift, and you leave it for the next shift to do its not particularly fair.
you try to be fair in life, there are good and bad ways for managers to do things, and maybe this wasn't the best but. as seems to have been stated, had you helped a couple guys wouldn't have waited an extra hour for a bus. its the nicety's we do for other people at work, and they do for you, that make for a wholely different work experience. you can work at a job where some people don't care, slack off, and the rest pick up the slack. or where you help people out, and they, when you need to get off 5 mins early to meet someone or get to the cinema early enough or whatever, will do you a favour.
i stayed late a few times, and if we worked stupidly hard or had an incredibly easy day then we went home early now and then and still got paid. that was yonks ago at M&S who were always pretty fantastic. i'd be at college, get up at 6.30, leave at 7.30, get there at 8.30, be at college till 5ish, travel an hour home then to work, work 6-6.30 till 11, that was my week. despite that i would still stay late if we weren't done, and we went home early if everything was done.
it would be very noticeable when some people where on the same floor as you as you'd always leave late, as they were slackers, but the management were very good at keeping them in certain roles where they couldn't slack off easily, on the tills on a sunday before xmas the queue of grannies wouldnt' let you be slow. good management will sort that out though, bad management will leave bad people in the wrong role making everything harder for everyone.
M&S rocked completely though, i got any extra hours i wanted, cash weekly pay, took me on for an extra couple months, was going to make me permanent but got really ill for 2-3 weeks. they paid me for 4 weeks work time off after i said i think the school, plus weather(my joints get bad in winter + being on feet at work was not good) plus work was too much so i left. they paid me for work i didn't do and were fan-bloomin-tastic to me the whole time. but then they knew i worked my socks off, worked harder than most, used my brains, put out new stock and made space for it, check price put up labels without being asked, it just made sense. if there wasn't stuff to do , i found something extra to do, and my manager noticed less work for her and thats why they treated me so well. with a fantastic reference and the offer to come back when school eased up/got better. treat them right, they'll treat you right is how companies should work.