Speaking as an ex trainer, ex Trainer Corp lead, frankly, people do not remember 15 minutes.
Now you'll not likely forget the hour anytime soon will you?
Besides, in the TA if you have a rowdy crowd the trainer would get nothing else done if he had to rewarn, reboot people every 15 min.
At an hour the people at fault have a bit more time to rethink their actions. The Trainer involved gets a breathing space in which to actually train.
Plus people have a chance to cool down before they come back in.
Otherwise you'd have hotheads wanting to continue the same arguement that got them in trouble in the first place, right?
Now you know why

Go forth & sin no more grasshopper!