What were the other employees doing? This happens everyday where I work and it's especially bad when changing shifts. If the other employees weren't doing anything then I can certainly understand where you're coming from, but if they were actually doing something (it may not look like anything that couldn't wait a few minutes longer, but a lot of times shit has to be done right at shift change), then just chill man. Again, if you happened to walk in during a shift change, a lot of places are unable to open the second register until the shift change is complete simply because that register is used to gather totals and print out reports for the closing shift.
I can tell you that standing behind that register is just as frustrating when you have that many people in line, it's very intimidating. A lot of people get very upset having to wait for even a couple of minutes. But these same people will go to a department store and wait in line for a much longer period of time without even so much as a subtle sigh.
It's just become expected that at gas stations and convenience stores you should receive almost instantaneous service. And believe me, when people get pissed off like that we want to get them out of there as soon as fucking possible.