Design Store, the lamp you have most likely has a label on it somewhere by Underwriters' Laboratories, indicating that it is "Listed". This means the manufacturer has brought his product to UL for testing and listing, and brought it as that it will only be used with a 100 watt lamp, for what ever reason they chose.
I suspect they chose 100 watt to avoid any product liablility law suits against them, in their belief that 100 watt is safer than 150 watt, or the max of 250 of the socket.
Why the socket for 250W? A combination of purchasing the most popular socket available at a very cheap price for the hundreds or thousands they need for their product, and a bit of over engineering, their thinking to use a higher rated socket, and derate it's use.
Also, keep in mind the more availabity of halogen lamps, which are notorious for beng used improperly and have been casues of fires, due to the high surface temperature of the bulb.