Someone said that it is because of "Realism" and "Safety". He explained that the avatar set's the firemode to Safety when you switch weapons and then when you switch back to the original weapon the avatar sets the firemode to the first mode after Safety mode.
In response to that argument I must say that if the game avatar is doing that without the Player telling him to do so, that means the avatar is has some sort of willpower of his own. If that is the case why doesn't he remember which firemode the player had commanded him to use?

P.S. As someone who has had military experience I can say that no one will set the gun's safety on, while switching to his side-arm in the middle of a firefight!

Also I believe it is very easy for devs to fix this predicament with a script (maybe 20 minutes) and allow the player to choose if he wants the game avatar to remember his previous firemode or just use the first firemode that is next to safety mode.