Someone needs to get fired over that one, and it's time like this that I think lawsuits should be brought. Not against the ambulance company mind you, but the man who was being treated should sue not only the company, but the person who physically put the boot on the ambulance and the person who authorized the boot to be put there.
The sign clearly says that your car will be booted if you leave the property, and they never left the property. That, and it should be common sense that if an ambulance has it's lights on, and they're inside treating a guy, then they're going to need to leave quickly. Putting a boot on their ambulance is such a dumbass thing to do, I honestly can't believe they did it.
Really hope they get fired and sued, they deserve it for that one.