If a player wants to sit there and fold every hand, the player is allowed to do so. So, what's the difference if he is there or not?
The answer is: not much, but there is one difference. I'll discuss that difference in a second. So, if a player can sit there, fold every hand until there are no chips left in their stack, and still cash and get points if earned; why would you not allow that player to cash and get points if the player is not there? There is no difference in that respect. There or not, the player paid the entry fee and is entitled to any reward. The player is also entitled to have their stack treated just as though they are there, even though they are not. Never should the stack be blinded off more quickly, simply removed, etc. They should lose chips no more quickly than if they simply folded every hand. IMO, not doing so and not giving the player any earned points or money is not only cheating but stealing
Now, the one minor exception. As
@BGinGA has pointed out, a player not being in their seat creates a positional advantage for the player to the right of the empty seat. Basically, that player gets the button twice. That's where the difference comes into play. If Player B leaves, Player A (to Player B's right) knows Player B can't raise or call and basically gets the button twice; whereas, if Player B is there and just folding every hand, then technically, Player A doesn't know if Player B will suddenly decide to play a hand. Therefore, while I used to be on the fence about this, I'm now an advocate of
@BGinGA's method of removing the players blinds when the button passes over them. It is fair and does not give a player an unintentional button advantage.
The counter-argument to removing blinds as the button passes is that, as long as the seating was randomly assigned, getting a "double button" is just luck of the draw. That's true; however, for most cases when it is "luck of the draw", things will even out over time; however, since this is a very infrequent occurrence, it is not likely that this type of luck will ever even out. Therefore, I don't agree with that counter-argument.