We took a helicopter flight and they did weigh you but not in front of everyone - it was discrete. The reason they do this is for safety. All aircraft have a weight limit. If the weight limit is exceeded, especially at higher altitudes, the aircraft can be at risk. Since so many people tend to fib a little on their weight, it is important to weigh them. Nobody wants the flight to be a one way ticket to eternity. On floatplanes it is not as important to be weighed because they usually are not filled with cargo, but they too have a weight limit. Commercial planes also have a weight limit and they will sometimes have to reduce fuel before take off.