How do the HS coaches out there feel about this?

Different rules for different states.  Good chance (I guess?) that it was ok so long as the HS season hadn't started.

I don't know why any coach (travel or HS) would be happy with a player wearing a uniform from another team.

justbaseball's response is very accurate.  If the HS season has not officially started, no harm no foul and not a whole lot anyone can do or say (to include the HS coach) to make a kid not play for his TB team.  The uniform thing...I would hope the TB coach talked to the player about that. 

Why would the travel ball coach let him take the field in the high school uniform?

Here is what I tell my players - once the season starts you are committed to the high school team.  From Feb to May / June you are not allowed to play on another team because there will be conflicts and it's not fair to either team for the player to miss one or the other because of the conflict.  Plus there is the added chance of injury from being on two teams.

I rationalize it like this - I'm only going to coach you from Feb to May / June.  You got the rest of the year to play for whoever you want.  I'm going to have opportunities in the offseason for you to get better but due to state rules they are not mandatory.  If I feel your travel ball team / coach is not getting you better I will tell you what I think.  But if you want to stay with them then by all means do so.  Just understand that if someone is better then you will get replaced.

Some people say I don't have the right to tell them who to play for but I see it as I'm letting you know what the requirements are to be on the team.  It's the players choice as to whether they want to follow those rules or not.  I know it's a semantic argument but all I want is Feb to May / June - the rest is yours.

I don't know what the thought process was with the uniform thing... I'm pretty sure it's a technical violation of state rules if one wanted to be picky about it.  I'll probably send a message to the school's coach without identifying the player so he can address the issue with his team.  No reason for the team or the coach to have to deal with the state association over a player's minor indiscretion.  I'm pretty sure the coach has no knowledge of it.

I think the thought when the league decided to allow 15U players was to have a place to play for those kids who didn't make their JV teams.  I don't think it was meant to really be a place for JV players during their HS season - but I guess if the coach allows it, they're going to play.