The main reason is any illuminated switch is expected to be driven by a lamp output on the OI. Those outputs are only able to sink 10 ma (milli amps) which is fine for LEDs but way low for lamps. Additionally, any operator box would receive power through the OI and by making this rule, it will keep teams from blowing fuses or killing the OI. As of yet, FIRST is not allowing separate power for the operator interface. I concur that with all the variables that would result in failures, this is a good general rule for all teams.
See Robot Control System p. 12 @
http://www.innovationfirst.com/FIRST...ence_Guide.pdf