Microwave radiation is only radiation in the electro-magnetic sense, your regular stove uses infrared radiation, a light bulb uses visible radiation
It also breaks down tissue to make it easier to eat - it's a lot easier to eat chicken casserole than raw chicken.
Sometimes the bacteria are in the source however clean the processing. Milk inside the cow can contain dangerous bacteria. You can get a variety of diseases straight form the teat, just as you could 100 years ago.
Pasteurizing is a cheap and easy way of partially cleaning milk - at least enough for it to be safe for two weeks. If you want it to last longer you can heat it more to kill more bacteria but you destroy more milk proteins and end up with nasty sterilized milk.
You can eat (almost) raw steak because unless the animal is badly diseased there are no bacteria in the meat, there is only a risk of contmination on the surface. The problem with hamburger is that you take surface and mix it right through the meat. If you start with steak and mince it (in a clena kitchen) just before serving it's safe = steak tartare.
It's the energy that the microbes absorbs - which is a combination of heat and time. The danger of a microwave is that the food might not have time to heat up fully all the way through and so the centre might not get hot enough for long enough to kill the microbe.