Not necessarily, no. See for example p9, which documents two famous cases involving full-time professionals. Good screening practices seem to help a lot, but as you know perfect security is an unattainable goal.
My point was not meant to imply they would accept bribes. They do the job to fight fires so why agree with the idea that we should stop them when some equipment was not yet fully amortized? If they accept bribes or not, the idea you stop volunteers from doing what they want (fight fires) is wrong.
Doctors are supposed to be moral paragons too, but every so often one is revealed to be abusing their trust in some horrific fashion.