If the piping is subject to freezing, per IPC, it must be protected.
Whether it is enforced - that is up to the AHJ.
This is usually an issue that should be addressed with the AHJ prior to installation to get their agreement.
I have a parking garage that I did - which the sanitary and storm is not insulated - and was not an issue when we discussed it with the AHJ.
Unless the garage is in an area where the temperatures can get really cold or can last for a long time, I don't think it is a realistic issue.
Even if the garage is open, since it is under a building, there should still be some heat in it. Even if the temperature gets down to below freezing at times for a short duration, the pipes are not full of stationary water - except the traps which you are protecting. The piping is designed to empty, if it is installed correctly, without dips or sags.
Talk it over with the inspector - maybe he will see reason. If not, then you at least have to insulate the piping.