What I want to say is:
Does opponent owe me something? No.
But does it cost anything to the opponent to grant an undo? No again.
So if it doesn't cost anything to the opponent, not granting is just being mean on purpose.
Of course they are allowed to.
And I am allowed to blacklist people on the other side.
Which, for instance, I do every time people do not grant me undo.
So this way I can choose which people I want to play with and which not.
Does opponent owe me something? No.
But does it cost anything to the opponent to grant an undo? No again.
So if it doesn't cost anything to the opponent, not granting is just being mean on purpose.
Of course they are allowed to.
And I am allowed to blacklist people on the other side.
Which, for instance, I do every time people do not grant me undo.
So this way I can choose which people I want to play with and which not.