Someone else mentioned this, but I'll repeat it. If all the people who withdrew had not qualified, it would not have moved the cut line and thus no additional people would have gotten into the virtual US Open.
In terms of Tiger, he wasn't the only player who withdrew. There are generally 8 to 10 players each week who withdraw for various reasons, he obviously gets the most attention. Back in 1991 (I think) John Daly won the PGA at Crooked Stick, I believe that he was the 8th alternate and had to drive up there the day before play was to start.
This was my 3rd major on WGT. I posted a horrible round in last years US Open at Oakmont. I got caught by the fringe on a downhill putt and made a 9 on a par four hole due to that, cost me four strokes and then just played badly. I came back with a 6 or 7 under second round to salvage a little of my dignity but I posted. The people who WD in the 1st or 2nd round don't post a score and finish in a tie for last, behind the people who post 80 / 80 for two rounds. Since there is no prize except the grand prize outside of qualifying there is no penalty for this.
We also might want to consider that maybe a few of them had computer issues that forced the WD, I have had it happen doubtful but it can.