That's a really good question that I have never thought to ask. I'm assuming you're talking about the 3-prong outlets which fit plugs with 2 flat blades (on 'top') and one round pin (on 'bottom').

The two blades carry the current, and the pin is the 'ground'. The ground pin only carries current when there is a fault in the system. A fault is when something disrupts the usual flow of electricity through the circuit -- if one of the wires gets damaged, or something metal touches an exposed wire. In situations like that, the ground pin will carry the extra current away from the fault and to the ground (literally) which should prevent all the current from going into the fork that you're holding. This is why the 3-prong plugs are safer than the 2-prong variety, and also why you tend to see them in devices that are used outside, or around water, or in hospitals, which are all places where some moisture is likely to get into the device, causing a fault, and potentially giving a nasty shock to anyone who happens to be touching the device.

So, why put the plugs upside down? The best answer I could find was on a message board for electricians, where one contractor suggests putting the ground pin facing up for this reason: if the plug isn't all the way into the wall (so that some of the metal parts of the plug are exposed) and you happen to drop something metal onto the exposed parts of the plug, it would be best for the dropped object to contact the ground pin (causing no damage) than the current-carrying blades (potentially causing a short-circuit).

I suppose they don't want to take any chances in the hospital! So remember to always be cautious around electrical appliances and wires. Also, even 3-prong plugs won't always save you, you can still get some rather nasty shocks just from touching the insulated plastic around a 3-prong outlet.

Because if a cord or wire were to fall down on a partially plugged in right-side up outlet you would short out between the hot and the neutral. When the outlet is "upside down" in the above situation the wire would touch ground first. Also I think it is code.

In fact, many years ago when I was an electrician in Connecticut, all receptacles in commercial locations were required to be installed upside down. I do not know if that is still the rule, but I suspect it is.

Back in those days, there were very few plastic faceplates on AC receptacles and switches. They were almost all made of metal. All that holds a faceplate in position is one short screw for an AC receptacle. If the screw loosens and falls out, the faceplate will drop down and short out the two power prongs on anything that is plugged into that outlet. Such accidents used to be commonplace which is why power receptacles were put in upside down.

Today, many faceplates are made of plastic although metal is still used quite a bit in commercial applications like hospitals.

I was reminded of this recently when a building in which I was helping to install data wiring had a fire emergency. While a person was removing a plug from a socket, the screw came loose and the plate dropped down onto the four cords plugged into the four-gang outlet. Two of them shorted out immediately and the other two followed suit a few seconds later. The breaker did not trip as it should have so there was quite a bit of arcing and sparking. By the time the breaker finally tripped there was a small fire going in the wastebasket below the receptacle and the metal plate looked like a blowtorch had been used on it.