I suppose I've left loaded and chambered rifles with the safety on on the range table, and I don't really see that as a problem. But the scenario is different as out where I shoot, I know who's there and it's hard for someone to show up unannounced.
This post does make me think that the small inconvenience of having to put a mag in and chamber a round when I want to pick my rifle up is worth the safety aspect, though. And it's a good habit to establish.
You're right and in certain circumstances that's probably fine - I shoot a lot on private property and in solitude so what happens there is quite different. At the range, my range in any case, it's inexcusable and the kid knew it as soon as he saw me too.
Cutting corners when no one is looking is a bad habit when it comes to firearms. Having said that, when I hunt and cull I work the bolt back to closed and pull the trigger, then insert the loaded mag and leave it in that condition - Impossible to fire. When I need to shoot I work the bolt and shoot. When done I take the mag out and do the process again and off I go. I never use the safety. At a range I'd never do that though.
Your last line...I think safety is always a good habit and at ranges a must.