They don't know you and they certainly don't know whether you are dumb or not. In fact you may be a genius and much more intelligent than us all. But, you might agree that you have not presented yourself as particularly smart on this thread. Here's why:
1. If you don't know their income, how do you stipulate they're being paid so much?
2. Have you considered that it's conceivable that people may be still be paid much without being "exposed to dangerous levels of radiation?" A better bet may have been to think that maybe it has do with the fact that their expertise is needed for patient care, they provide valuable services which requires a lengthy training and lots of medical knowledge, utility of their services has grown exponentially, there is a national shortage of them, etc. Essentially, a lot of things to consider before tying radiologists' income solely to dangers of radiation. If a junior high student had asked that question, people wouldn't have been tough on him/her, but I guess they expect more from a med student.
BTW, it seems that Stephew is a "radiation oncologist" rather than a "radiologist" (see the signature below his post if you didn't notice). Now, if you don't know the difference between these two types of physician, we'll be happy to explain that, as it is a very appropriate question for a medical student looking into these two seemingly similar fields.