In particular, the one-line system might significantly increase the average distance between (first-in-line) customer and nearest free cashier. Also, when there's a lot of cashiers, the customer might take more time to notice where is the nearest free cashier. I saw that happening the last time I checked-in for a flight.
In contrast, at immigration there is a long, single line but also short lines (1 to 2 persons) in front of each "cashier". These work as "caches" that minimize the delay I mentioned above.
The main goal is to prevent a single "price check" from jamming an entire line of people -- keep the queues at each station as small as possible so that most customers can route around the blockage.
http://web.archive.org/web/20100324183932/http://www.coinsta...
Other instances of "people are stupid" leading to poor design:
* Some cars come with a CVT (continuously variable transmission) rather than having to shift gears. The audible pitch of the engine just gradually goes up as the accelerator is depressed, rather than revving up and then shifting back down again. Since people were used to the gear-shifting behavior, they thought CVT's were underpowered, so they were actually redesigned to simulate the shifting behavior even though it was suboptimal.
* Coinstar machines are actually much, much faster than they appear. But people don't trust them if they don't take three times as long and make jangly coin sounds, so it just silently sorts the coins as fast as it can and plays a recording of jangly coin sounds, all the while delaying when it displays the final count onscreen.