[ErichKeane]

The battery short issue is easily solved by using fuses and circuit breakers. For a competition like this, I've been very surprised that there isn't a rule requiring inline fuses on the positive source lines of all the batteries. Otherwise, the fuses will prevent a short from causing any issue.

As for the difference in potential between two robots, it would cause no problem whatsoever, since only grounds would touch, so there wouldn't be any power loop.

Think about this example: you have a car that needs a jump. What is the first step? You connect both grounds on the batteries to eachother. In a jump situation, the difference in potential is going to be much greater than 3 VDC, and yet there is never a problem.

I will connect negative terminals of batteries all day, there is no danger.

By disallowing the grounding of the frame, FIRST is causing an even more dangerous problem, the situation where a single positive wire contact would could give the entire frame a POSTIVE charge, which, through a human resistor, could cause extreme physical damage.

There is a very good reason all cars have their bodies/frames grounded, it is for the obvious safety benefits.