Here's the deal: guns can only go off when there are three things: 1) guns, 2) ammo, and 3) someone messing with them. That particular rule is stop people from messing with guns while people are downrange. People were downrange and you were moving a gun around. You broke their rule, end of story. So fess up and take it like a man.
I certainly understand why the range officer lost his patience with you when you started to argue with him. The rule says no handling of guns while people were downrange -- 1) there were people downrange and 2) you were handling a gun. He told you what you did wrong, but you didn't accept it. It's no surprise to me that he raised his voice.
Different clubs have different rules. You may or may not agree with the rule at that club about not handling guns when people are downrange. But the fact is, that is their rule. It doesn't matter that your gun was unloaded when you took it from the truck. The guns benched on the tables or in the rack were unloaded as well, weren't they? Accidental discharges almost always happen with guns that people thought were unloaded. Which is exactly why many clubs have this rule.
As long as no one handles a gun while people are downrange, then there is very little chance of a gun going off. In fact, at many clubs not only can you not handle a gun while people are downrange, but you also must step back behind a yellow line, 5+ feet from the benches. This type of rule makes it much, much easier for a single range officer to supervise a large range.
Next time have your rifle in a case. Don't uncase it until you've asked the range officer if you can bring a rifle onto the line.
You may not like it and you may not agree with it. However, that is their rule and you broke it -- obey it or find a different club.