Without getting into the actual health and risk aspects of the virus and leaving this discussion with the law itself, if the rabbonim really felt it was such an emergency, they could make arrangements out of state instead of violating the law. Your rehab example fits this perfectly, as most people are sent to rehab out of state, even away from regular Jewish infrastructure, if the emergency warrants it.
The rehab example may warrant being outside a Jewish infrastructure to save a person's mental, emotional, and physical wellbeing.
However, to save a kid's emotional AND spiritual well being they actually would need to be in a Jewish environment. Unfortunately not all camps were logistically able to swing going out of state even if they started planning months ago, the earliest could have been late March which is not early enough in many cases.
My child is lucky enough to be attending a camp that moved to PA, I'm just feeling really terrible for the thousands of teens who were going to attend camps who were not able to move out and now they have nowhere to go because of a dictatorial decree.