This is the part that people are facepalming.
He is taking oppurtunities from no one.
Yes the 100th ranked player should be 99th but he's still the 99th player in line to get entry into tournaments since Roddick isn't playing so no oppurtunity lost.
Yes like you said the player should notify the ATP in writing when they want to retire, this is something Roddick has not done, he hasn't put it in writing like Ljubicic did when he retired and was immediately removed from the rankings.
In this day and age a verbal agreement is rarely binding, even though it should be. Roddick has not put it n writing that he is retiring, maybe he wanted to leave himself open to maybe playing this year sometime if he felt like he missed tennis I don't know.
If Litotes is correct and Roddick has sent his retirement in writing to the ATP then he will be removed shortly as he should.
A player has the right to say he's retiring all the while keeping his deserved ranking if he wants to come back. When said player decides he doesn't want that ranking then he has the right to remove it.
Boyth scenarios no oppurtunities are lost so who cares
maybe he was shrewd enough to have a sponsor contract where they have to keep paying him so long as he's top 50.
Can you imagine, milking it for a few extra months on a technicality?