Actually, most are designed that way to set up a circulation of the air to disperse the heat more evenly throughout the room. The cold air from the windows is heated by the radiators, which then rises, eventually cools and the process begins again... It's much more efficient that just radiant heat alone...
Its because its the best place for them, put mine on the opposite side to what my window is (most conveniet place at the time) but now wnat to knock through and cant because the bloomin rads in the way