I understand how you all feel. My son would have been really upset if his Pcoach left before he got there. He didnt, but son would have been ok either way because his college experience was about a lot of things, not just his pitching coach, but you don't realize that until after the fact.
Volunteer coaches are unpaid assistants that have no employer benefits but do make a salary based on the camps. Their salaries range from 25k to 40k depending on the program. They are learning from their HC so they can move forward when a paid position with benefits becomes available. They cannot recruit. I know of an assistant volunteer coach whose wife works at the same university and she has the benefits. NCAA only allows 3 paid school employees, along with a baseball operations manager.
There will always be movement among coaches changing programs for various reasons, especially for those with young families. Trust me when I say that it does bother them to leave their players, but just a part of the profession that will never change.