I think the cautions are more from a driver development standpoint. Learning to drive on street tires forces you to be smooth and your deficiencies as a driver are more apparent. When a new driver jumps immediately to R-Cps or slicks, they lose out on that development and driving errors can be masked by grip. That's one of the reasons why most HPDE organizations don't allow R-comps or slicks in green run groups.
Wow I have driven with a few different organizations and don't recall any rules disallowing any particular tire type for any group. I personally prefer to train on same tire all the time so I get max time with the tire.
I can see why they might caution folks that are new to their car about running slicks, especially when it comes to moisture on the track. If you don't know your car well you probably will do the wrong thing when this happens. Of course the best thing to do is go back into the pits and switch tires ....