I actually hear about these "showcase" "high school" conflicts a lot. The biggest most often gripe I hear is the high school coach that will not allow his players to compete with travel teams during the summer and/or fall.
I just don't know of many travel teams that get in the way of high school baseball season. In fact, most of the travel coaches I know are big fans and supporters of high school baseball. There are some that cause issues, just not many.
I believe kids and their parents should be able to do what they want if it does not conflict with their school team. Nothing can stop 6 kids getting together and playing "hard ball" on the play ground. If they get hurt who do we blame? When something like a concussion happens, everyone's main concern should be about that injury. Not whether it hurts the HS team or hurts the travel team. We simply listen to what the medical people say! They don't care about either team, they only care about the person injured.
I don't know about other activities. I do know my grand daughter is involved in cheer leading at her HS. She also takes dance classes with a dance club and competes in dance contests. Her HS coach actually promotes it. Guess her thought is it will make her a better cheer leader. The two activities keep her very busy, but she is also a straight A student. Granted she hasn't suffered any serious type injury. She is very good at all that stuff. I actually enjoy watching the dance competitions though I don't go to many. I'm too old to watch the cheer leaders when a game is being played, though I know a lot of work goes into it. She also competes in cheer leading competition, but I haven't gone to one of those yet. I know if she suffered a concussion, the doctor will dictate when she returns to either dancing or cheer leading. She could get injured at either one.
Anyway, in the case that "coach2709" posted about, I agree with him about the coaches and also those that are blaming the girls parents. If your kid was not allowed to participate in HS activities due to injury... How can you possibly allow her to participate in the same activity outside of high school. Especially when we are talking about a concussion. If the coach is allowing this, knowing she is injured and not allowed to participate in HS... He is an idiot! Worse than that... He is a dangerous idiot! But for sure her parents know! Maybe they just don't understand. That is the only excuse I can think of.