Nearly every other MMO I've ever played has an auto-afk kick mechanism. Frankly, trying to argue against one is just pure selfishness. Everyone is paying the same fee so quite simply somebody who actually wants to play should always take preference over selfish individuals who are taking up server resoures while they work / sleep etc.

Make it an hour rather than 20 minutes perhaps, but trying to defend the practice of taking up an active slot while you are deliberately afk for an extended period is simply childish and illogical when the practice of auto-afk disconnection is so commonplace in other games.