At the risk of beating a dead horse, I'll try to answer the questions above:
I do think most teams could function with an occasional absence. I also think most coaches will, if approached in the proper manner, consent to many advance requests. But if a player simply misses and then gives his reason after the fact, or if the player speaks up in advance but presents the matter as simply giving notice that a decision has already been made and the coach, having not been asked, is merely being told how things are going to go, I think you could well see the coach act so as to make a point -- even if the circumstances were such that he might well have given consent had he been approached correctly. The coach has to be clear about who's in charge and who's calling the shots, and to that end, he may have to administer an object lesson so as to avoid having his authority undermined. It is also true that sometimes an absence will really cause a problem, so it should be understood that even properly presented requests will sometimes be denied.
The excuse that "family comes first" or "family lasts forever" does not justify making a commitment to a group activity, then reneging and expecting the person in charge to deal with the consequences of your whims whenever they strike you. That is really just being a prima donna and then hiding behind high sounding language.
Look, if you truly want your family's activities to come ahead of baseball, you absolutely have the right to make that call. All you have to do is say so up front. But you should also understand that if your priorities don't line up with those of the people running the show, you might lose out on a roster spot, to someone who was ready to be there 100% of the time. The problem here is that people seem to want it both ways: They want all the benefits they see from being on this particular team, but they don't want to accept the full commitment that this particular team entails.
Or perhaps they just don't understand that standard rules of conduct, good manners, and the respective roles of player (subordinate) and coach (superior) are all in play here. So they make vacation plans during season, they join the team without saying a word, and then they expect the coach to organize his plans around their absence on short notice. There's a lot of "me" in there, don't you agree? Think of it this way: If you went into your boss's office at work and said, "I'm going to be out Thursday," how would your boss respond? Might it go differently than if you had gone to the boss and asked, "May I have Thursday off?" If you understand the difference, then at what age would you contend a person should learn the distinction? Personally, I should hope this lesson gets learned BEFORE a person's got a job at risk. Meaning, it is wholly appropriate for a coach to administer this lesson to a teenager, as opposed to indulging the player because "he's just a kid."
It's definitely true that not everyone has baseball as their top priority, but the OP specified that he is intent on pursuing collegiate baseball. The translation of that is that baseball is supposed to be HIS top priority outside of school. Someone seeking to get recruited doesn't put a beach week ahead of baseball, not if he's serious about it. Every game is an opportunity to be seen, and every practice is an opportunity to prepare to be your best at the next game. You can't give away those opportunities lightly.
As to what a player could expect from a coach if he failed to make the advance request and merely told the coach he was not coming (without any thought that the final decision was actually the coach's to make), then I think the player should expect some reaction. The typical response is not suiting up for a game or some such.
But the other question then becomes how the player responds. Does he learn the proper lesson, apologize, and conduct himself properly going forward? Or does he resent not having his way, or resent the consequences he faced, and then badmouth the coach behind the coach's back? If he learns his lesson, we should hope that the coach becomes someone who would speak well of the player in the future. But if the coach learns that he's being badmouthed behind his back, small wonder that the player and he don't see eye to eye, and in that situation, the player has squandered the chance to cultivate a key reference and thereby damaged his chances in the recruiting process. Because I would surely expect a college coach to ask the Legion coach for his take on the player before committing to him.