My reasoning is that some people abuse it, yes, those people are an issue. But its 2-3 minutes, its not gonna kill you.
I bet you are young, 16-22 years old. Life is too short to waste 2-3 minutes idle in a game.
For ex. I've seen shadow to wait those 2 minutes in 4v4 and than he killed one of the enemies who were left so we won the round. Was fun to watch.
I doubt the purpose of an arena was to let a team win with first skill after having spent 3 minutes doing nothing. Also, I doubt it is right that a single stealther healer should win against 2 or 3 enemies at full health just because that player hid away for minutes and survived sudden death 1 sec longer.
Anyway, thank you for your responds, now that I realized I should expect stealthers to do this more and more often, I will no longer play arenas.