Thank you for the compliments on Inside College Volleyball and I appreciate you recommending the site to parents which are looking for help!
Sand volleyball is an interesting creature - It is all the rage in some circles and in others it is not viewed positively. By my writing, you know that I feel collegiate sand volleyball is currently a gender equity vehicle for athletic departments and increases the time demands upon indoor coaches and players. Only a few schools have actually developed autonomous sand volleyball programs, and if they are a 'real sport' then sand volleyball should have nothing to do with indoor volleyball.
It does not surprise me that many high school and club coaches are not supportive of sand volleyball - It is a different sport and the skill sets do not all crossover positively.
As for why club/high school coaches may not like sand for their players, for indoor volleyball you want a long broad jump in your attack spike, and for sand volleyball, the broad jump is minimal. Setting is another example - Sand setting you can grab the ball, autograph it, dust of the sand and then release it; this does not work even in the ultra liberal contact rules of indoor. In fact, when I played on the AVP Pro beach tour, I never hand set because it messed up my release for when I went back to playing professional indoor volleyball in Europe.
The blocking is different, the passing is different, the defensive movement is different - It is a different sport AND if it is played by FIVB rules, the court is also smaller!!!!
I can see HS/Club coaches not liking this because if they have been working on specific indoor skill sets pertinent to their team's success, and a player goes into the sand environment and these indoor skill sets suffer, then......
Now, that being said, I do like sand volleyball for a player's all around game and teaching competitiveness. It is also a great conditioning vehicle, which allows for passive strength gains with out having to suffer repetitive stress situations.
And now I will dive into the deep end of the pool - Think about what the indoor game has become; it is a game of specialization. Very few players actually play 6 rotations anymore; three rotations of play is the new normal - The implementation of the Libero, removed any back row skill demands from MB's, the increase of the sub counts means that OH's/RS's can play a whole career without ever passing or playing back row defense, and when NCAA went to 15 subs, many programs went to a front to back 6-2 offense where the setters never see the front row either. The Liberos spend the most amount of time on the court now....who would've have thought?
So, from that stance, it makes sense that the club/hs coaches don't want their players practicing an all around game, on a surface and court which is different than indoor - They want their players focusing on their specific skill segment applicable to indoor success.
This is one of the situations where, "don't hate the player, hate the game".