First time I ever saw this.
Since I was not 100% sure I should have ignored it.

So here's what hasn't been said, even though you only asked for a rule cite.

Most tournaments and most home/host teams do not allow the infield to be used at all pregame once it has been "dressed" and made game ready, because they do not have the staff (or choose not) to redrag and reline the infield. Even in NCAA, where they DO have the staff available, they generally only allow "lines" in the back of the infield cuts, rather than an actual "infield", then rake out or drag only those areas.

By tradition, what you stopped is generally not allowed by game administration for the reasons I have stated; but there is no "RULE" in the book, just what the game administration allows (or doesn't).

Thanks.

A number of leagues and tournaments specify in their local rules that neither team can use the infield to warm-up before the game. It makes a mess of the infield and makes it hard to keep games on track when there is a tight schedule.