This i can agree with. Making the change like that is a fair one, though it might target those, who chooses to not disable it, which would be a bad thing. At the end of the day, if all that bothers you is people saying welcome back, I doubt that is going to change.
In honesty, the much better choice would be for us to be allowed to actually AFK our selfs, and have a message showing that we are afk if people message us, so that we're not being concidered rude or blamed for ignoring people. Having the /afk for staffs only is unfair. We all have lives that can be interupted by things.