I've no interest in leaping to thr defence of FR, however "it's too noisy and it's in our rules" is the most frustratingly disgraceful comment to be made by a person who works on the ramp of a commercial airport.
I appreciate your frustrations, but it seems to me that you've been subjected to a person who failed to communicate properly. Several airports do have restrictions on the use of APU (e.g. 5 mins after on-block and 10 mins before off-block), and that's nothing to do with the handlers. If is, however, their ears which will be suffering in the end.