Tulo,
I've umpired high school baseball in multiple states and can assure you that the "around here" where umpires don't grant time outs to facilitate throws to the mound includes a lot of territory.
Unnecessary time outs add unnecessary minutes to games. They can affect the fairness of competition. And they retard the development and game awareness of players. Good umpires discourage them the same way we require batters to stay in the box when they want to wander off to slow things down and gain an edge.
At the high school level, the offense and defense are expected to protect themselves or the ball from the conclusion of one play's action until the attainment of what we call relaxed action, which is a state where the live ball is under control and neither team is likely to try any surprise funny business because the other team would easily thwart it.
Teams are expected to be able to return the ball to the mound without needing a flag of truce to accomplish the job. If they can't manage the job without throwing it away or getting startled by dancing base runners, I won't help them get it done.
When I'm presented with a request for a time out whose need isn't readily obvious, the factors I consider include:
--Whether the time out is intended to facilitate a baseball action that should be accomplished without the aid of a time out or a non-baseball action. Baseball actions include regaining one's feet after sliding into a base and returning the ball to the mound. Non-baseball actions include shaking the dirt out of one's pants, delivering protective batting gear to a coach, and meeting on the mound to discuss strategy. I will generally grant time for appropriate non-baseball actions, but only after the baseball action has been completed.
--Whether either team might gain an advantage from the time out. The very act of requesting a time out for a baseball action tells me the team thinks the time out would benefit them.
--Whether the non-baseball action can be accomplished without a time out. I've lost count of the times catchers requested time to visit the mound with no runners on base and I assured them the batter is unlikely to steal first base.
Finally, as one of my brethren pointed out, baseball is a continuous action game. The ball is live except when there's a good reason for it not to be live.
I've had conversations on this subject with lots of coaches. My usual explanation is something along the lines of, "Coach, at this level, the defense needs to get to ball back to the mound to show me the play is really over before I can grant them time." It's never been a big deal. Some mild surprise, but no outrage.