I know it's by player choice of course, but it's the ATP's computer, other sports when players retire remove said player from their stats etc. so why not tennis, and you can't expect the ATP to wonder if that player will come back, which they usually probably won't, they can't be dealing with that, as I said before it's not fair to active players to have retired guys taking up their spots, like for instance when Roddick made a big deal of his retirement, I think it's quite certain he won't be coming back, so there is no point in having him still in the ranking.
When is a retired player removed from the computer btw, when the season ends?
Why is it "not fair" to active players? Where's the harm? It is no worse being ranked #102 behind five retired players than #97 behind none. You compete for places in tournaments only with those who want to play said tournaments.
Retired players fall off when their results do. Sampras was ranked #34 before the start of US Open 2003, then afterwards he was gone from the list - a full year after he last played. The same goes for those who have publically announced they were going to retire, but stopped short of asking to have their ranking removed.