There are times when it's OK for a coach to talk to another coach's student, and times when it isn't.
I look at the PSA as a trade union, and myself as an employer. Trade unions and employers often don't see eye to eye on all issues. On top of that, sports organizations are sometimes allowed to bypass the rules on anticompetitive practice that affect other professions.
Plus, it isn't altogether fair for one coach to quietly undermine another coach's position with their student or their student's parents without giving the second coach a chance to defend themselves. I think PSA rules are partly an attempt to make coaches play fair.