Controlling a player lets you chose what to do with their commander if it dies/is exiled
Controlling a player lets you chose what to do with their commander if it dies/is exiled. E.g. after opponent cast Emrakul, the Promised End, they used my own kill spell to kill my commander and then left it in the graveyard.
This is against the rules - 903.9a. If a commander is in a graveyard or in exile and that object was put into that zone since the last time state-based actions were checked, its owner may put it into the command zone. This is a state-based action
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[Deleted User] commented
That check happens only one, the next time state based actions are checked after the commander has hit the graveyard. At that time, your opponent controls your decisions, so he can make the decision for you to leave it in the graveyard. And then that's it, nobody gets to choose again later. For that, another zone change would have to happen for your commander. The very rule you quote proves, that that was correctly handled.