It's to prevent people with wet hands from electrocuting themselves. In the UK it's required by law (building regulations BS 7671), that electrical switches have to be either outside the bathroom or specially water-protected.
How realistic of a threat is that? I've never heard of that occurring in the US where there's a light switch in most bathrooms--and if the news could make fear out of it they would.
Probably just a very old regulation, from a time when that was an actual serious risk, that's stuck around out of old habit.
Depends on the size of the bathroom (at least in Canada) - you must not be able to reach the switch and the water source at the same time. Or use a GFCI protected switch.