You can't really compare players and coaches in this regard. Players contracts are bound by the rules on the NBA CBA and hence, if a player just wants to not play for a team, he has options to get out of town with, like retiring(Rasheed Wallace), forcing a trade(Carmelo Anthony), coming to an agreement with the team to get out of the contract(Darko Milicic) or just leaving(Allen Iverson) and hope the team doesn't sue him over services not rendered.
Coaches are not bound by the NBA CBA. They have personal contracts with the teams and it is common practice, if a coach wants to not coach a team anymore, to let them go. it happens all the time(Larry Brown). If Doc leaves he is no different than a whole bunch of other coaches that just leave their teams for front office jobs or other coaching positions or to take a break and go into broadcasting. Seldom do coaches ever reach the end of their contracts. They are usually fired or they leave before their contract ends. And that is another difference. Coaches can be fired, players can't.