What you really mean is not "these professions can do no wrong", but "the media finds it difficult to blindly tar these professions the way it does to others".
Doctors can do no wrong? Is that why malpractise has help skyrocket the cost of medicine in the US? Doctor shortage a machiavellian plan to keep salaries high? Pull the other one. Yes, I do think that there aren't enough people with the smarts and the motivation to be a doctor - in fact I think that there are less of them than actually are doctors.
Policemen can do no wrong? Yes, in law courts they're seen that way, but I can guarantee you that plenty of people see police as doing plenty wrong. Police cop plenty of abuse.
And Teachers can do no wrong? Tenure or not, teachers cop it from students, administration and parents.
Firefighters can do no wrong? Meh, firefighters have the soft option. Cool toys and generally don't have to deal with the general public if they're not paramedics. Stick it to 'em :)
But really, the 'can help friends in Katrina' argument you make is just desperately clutching at straws. Not sure how this is a prime motivator for emergency personnel (I know a few, and this is not why they work in their respective forces) or as a 'can do no wrong' argument.