I've often thought that seemed pretty implausible. Well, elements of it are plausible.
A death ray -- a laser or phaser or whatever -- that works by burning someone up would likely affect a person's body but have very limited effect on the air around him. Consider what happens when you open a hot oven to remove, say, a cake. If you touch the sides of the oven, or a metal baking pan, you'll be badly burned. But you aren't afraid to be touched by the hot air in the oven, even though it's the same temperature as the metal pan. Air is so much less dense than metal that even though it's the same temperature it contains very little heat, i.e. very little actual energy.
If he's standing on dirt or rocks, the heat ray wouldn't have much effect on them either, as the heat conductivity of dirt and rocks is also pretty low.
If he's standing on a wooden floor, I'd expect the floor to catch fire, or at least that there'd be a circle around him that becomes charred and blackened. If he's standing on a metal floor in the starship, I'd expect the floor to become hot and everyone standing near him to at least have to be dancing as their feet are burned.
Likewise, if he was sitting on a wooden chair, I'd expect the chair to be burned up along with his body. If a metal chair, it should at least be glowing a nice cherry red. If a plastic chair, it if doesn't burn it should at least melt.
But I presume the real reason is that the producers are trying to maintain a PG rating. If someone really invented such a death ray, I suspect that what would really happen when you used it is that you'd burn a hole in the person you're shooting at, and he'd have burned guts falling out of his chest and in general be a pretty ugly, grisly mess. Or if it could really vaporize a person, that it would leave charred stumps of arms and legs behind. That would have been way to ugly to show on TV in the 70s and 80s. Maybe today it would be acceptable. But still, they probably don't want to show the heroes killing people in an ugly way. When the hero kills someone it usually has to be neat and clean. Also, I'd guess the special effects for believable burnt leg stumps is more expensive than a glow and the person disappears.
Personally, I often wonder how when a person is thrown back in time or travels to another dimension or whatever, he's always transported with his clothes and whatever he was carrying, but he never seems to bring along a chunk of the ground or the floor or anything else around him. I recall one science fiction book I read years ago -- forget the title -- that had the hero thrown into the future by being caught in a beam from some experimental machine, and the author threw in a line about how he was lucky that his whole body was caught in the beam, but it did leave behind the heel of one shoe. I suppose as the physics of such things is all made up, one could invent some equally made-up explanation.