The way refrigerant works is that you get cooling where the liquid, under pressure converts to gas at a lower pressure. So you have higher pressure on one side than the other.
Bottom line is you are right that you should not be getting frosting at the compressor, at all.
There are a number of reasons for getting frosting in different places on different lines.
If it is the high side line coming out of the freezer you likely are overcharged and the remaining liquid is evaporating after leaving the evaporator.
Most common it is the high side line leaving the compressor. Then you have a bad compresser (low pressure), a restriction at that point, or a leak or undercharge (gives same low pressure symptom). But if this is the case your compressor would be running 24/7 and it would be cooling poorly or hardly at all.
If it is at the dryer then you have a plugged dryer or he welded in a restriction at that point.
Without knowing exactly which line and where it is frosting it is hard to tell you exactly.
I would call the repair company, ask to talk to a supervisor, explain what you are seeing, and ask they send somebody different to check it out at no cost. If they refuse or blow smoke, I would say you are going to call another company and if they find he made a mistake you are going to want your money back for the second service.
That should at least get you a decent response.
One problem is that don't give the techs the time to do sealed syslem type calls if the are not expecting to do them. So techs do blow smoke at times if the problem doesn't look serious or they think they can put you off. Plus mistakes they have to do for free.
Good Luck.