FED pregame warmup pitches
 

I have a gentleman insisting a pitcher cannot warmup on the field using the pitcher’s plate prior to the start of the game. He is talking about players on the field like 15 minutes before the game. I don’t see any rule other than no balls hit or thrown in live ball territory during the pregame conference (4.1.2]. Am i missing something?

There is an instruction for pro baseball umpires that the home team cannot warm up on the mound pregame. It’s an unfair advantage. Might be an etiquette thing in softball.

This is not an umpire issue.

It shouldn't be, but probably can be if s/he is the sole non-team official involved.

Using what rule to stop it?

Here it is:

Most, if not all, state high school leagues have a handbook that covers down on policies and procedures regarding all of their sports and other activities. In it, there's usually a section about sportsmanship, and what constitutes violations. If a school allows their pitchers to warm up from the game's pitchers plate, but won't allow the opposing team to do so, that would be a problem that is covered under the state sportsmanship rule or policy.

But if both teams' pitchers are allowed to do so, what's the big deal? The teams do their pregame drills on the actual field, so why can't the pitchers warm up from the actual plate?

So in answer, there is no rule preventing them from doing so. As long as both teams are afforded that opportunity, it's a non-issue.