The concept of retainers was probably developed from what they learned from XI. Many people would AFK in Rolanberry and get killed by a trained Simurgh, (lulz) and later on they would AFK in a part of Jeuno depending on server. They also realized that people have a ton of mules so they made them cost $1 each, (Same as an extra character in XI.) instead of the $3 per new character for XIV.

AFK detection is rather simple. Flag someone with the away icon if they are idle and don't do anything for about 10 minutes. All you have to do to remove the flag is move a little. If the away flag stays on for more than 15 minutes, boot them off.