The pin is long enough into the axle, and it fights tight enough, and there is enough friction, that there is more force keeping the bike on the pin than force trying to slide it off, thus it can't slide off.
mitt
i wouldn't say can't, anything is possible, though it is not probable. i have had my bike move on rear stand before, but i was trying to slide the rear stand sideways to get the bike tight up against a wall. i saw it slide off about a 1/2 inch so i just grabbed the tire and put my foot against the stand and pulled it back into place, no biggie. but when used properly the rear stands are very secure.
the t-rex stand actually has a treaded rod w/ a big washer to lock the stand in place. i think it is a bit of an over kill.