The
USPSA rules are here in .pdf format. A cursory reading doesn't shed any light on the issue, but in my experience, everyone at a USPSA match is supposed to have a cold handgun until they're in the shooting box and told to make ready.
The first time I went to a USPSA match, I was taken aback at how stringently they pressed safety, but I understand that not everyone at those matches is a professional. There are a lot of amateur, and a fair smattering of absolute newbies. We want to bring those guys along in the shooting sports, and safety is the best policy when we're dealing with firearms.
Sorry I didn't answer your question. Their range, their rules. I'm okay with that. When we go to my range, we can play by my rules.