My take is the principal made two errors... jumping to the conclusion that the kid is getting "too much" screen time, as mentioned already, but also reacting to Dreambox as a threat, because he knows that the further your DS progresses in math on his own, the more problems it'll cause him to try to meet DS's level. So, he chose to "advise" you on what's easiest for him... unplug DS so he stops learning.
Even if you clear up the rushed conclusion, you still have no interest in helping the principal out of his problem, and he's still going to be upset about it, so I don't see how revisiting the issue is likely to be productive.
Just my take.
