Microwaves

Question

I have noticed that most Orthodox people use a microwave just like an oven. That is, they put the food in and turn it on, without any covering, whether it is dairy or meat. If I am at someone's house and I see that they heated up a dairy item this way for lunch and then for dinner they heat up a meat item without covering it, can I eat the meat item? Also, why do you mention only covering the food? What about the sides?