I just found the idea of a 300MW laser quite funny...no hard feelings!
It all depends on how long that kind of power is output. High peak power lasers that are table-top sized were able to put out 10-100 times that much power more than a decade ago, but only for a very short period of time (femtoseconds).
You probably could trash the cameras with a laser, yes, without trashing the phone, but it seems like an awful lot of trouble. It would make more sense for Apple and others to just cave and make "government" versions available without cameras like, as others pointed out, is apparently already done for the computers.