Because most people won't use a rifle as a defensive weapon. It's either going to be used for plinking; which is what they're doing; or for hunting. And in hunting, most rifle hunters brace their rifle on a bipod, tree, fence post, etc... So, for the average shooter, they ARE PRACTICING. They're practicing shooting a still target. At the most, possibly practicing for hunting.
Then you have the other extreme. The ones who are so into the competition/marksmanship of long range shooting. They think because they can rest a rifle on a bench, vise, etc... and shoot a target at 300+ yards, that they automatically are going to be a great home defense shooter. You can tell a lot of these folks, because they LOVE to use the letters "MOA".
The reality of home defense, is to hit your target in the center of mass. This is about the size of a basketball. If you can hit a "Probably Moving" target, inside of 100 yards; "More likely inside of 10 yards", in a 15-18" area, then you've done what you're suppose to do. But again, most home defenders won't use a rifle for home defense. Either because they aren't comfortable enough to use a rifle, or because in their particular scenario, a rifle isn't a good choice. Yes, you'll have the AR/AK hard-core owners try and tell you that it's the most effective self defense round there is. Effective is relative to your environment. For many people, a rifle is the wrong weapon. In my house, a rifle is the last weapon I'm grabbing for home defense. My primary is a 357 magnum revolver if it's 2am and I'm in bed. If I'm awake already, it's a Sig p220 45acp or possibly a 9mm. I don't even listen to those who want to argue using a rifle and not using a pistol. Most aren't worth my time.
So, you need to find out what those people at the range are planning on doing with their rifle. If it's practicing for hunting or just target practice, there's absolutely nothing wrong with sitting at a bench. If it's for home defense, then they should practice various tactical positions. But realize, defending your home is not the same as an assault team going into a building and clearing it. For the true home defense, there's nothing wrong with someone preferring a pistol over a rifle. I love all my AR's and all my AK's, but they would definitely not be my primary defensive weapon.