Only just seen this...
It is a good rule imo. You have a responsibility to your fellow players. If it is 4 handed to the river, player 1 has crap, player 2 has crapper crap, player 3 has poor, but some showdown value, player 4 has nut crap.
If the hand is played out properly, player 4 is less likely to take a stab at it, because he has to fear a call (or CR) in 3 places.
If player 1 and 2 "check-out" by open-folding, now when player 3 checks, player 4 knows he only has to get past one player and so is more likely to bet.
Player 1 and 2 have taken no positive action to influence the hand (i.e. by betting or something which is of course allowed!) but they have through their actions put player 3 in a tough spot more times than they should.
Player 3 is entitled therefore to some protection from player 1 and player 2 by having those players in the hand (and not folding, or making it obvious to player 4 that they will be folding)
I give that protection/courtesy to others, and I don't think it is too much to expect it in return.
Very good reply explaining why it is unethical to open fold in multy way pots. It happens often though I'm sure most do so innocently with no intention to influence the game.
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