This is done by sending a very mild electric pulse of a super low voltage across a series of wires ... so low that it does not harm the bees. It simply agitates them and induces their sting response, so in attempt to sting the wire, they deposit a tiny amount of venom on the glass. Their stinger is not caught on anything, it simply touches the glass.

The key is that they do not sting into anything soft or porous - like flesh. Therefore they do not lose their stinger... which would cause certain death. Instead, this process is more like the milking of a snake.