These are not laptop computers, they are notebooks. The only laptop I've personally seen had a 80286 processor... We call them laptops, but historically that name is wrong.
While it is possible to use a notebook on your lap, you are not supposed to. It is a terrible place that is unergonomic. You are supposed to put them on a table of some sort. If you are using it for more than something quick you should have a separate keyboard and mouse (ie attached to a docking station on your desk). The portable form factor is useful for meetings, presentations, or other such - but for real work they are terrible.
Since the lap what it was designed for, and a lap is a bad idea anyway: putting vents on the bottom isn't bad.
I frequently sit on my balcony, with my MacBook Pro in my lap. I'm still alive.
I also frequently use that computer, that doesn't have vents on the bottom, for watching movies in bed. And I don't need to think about vents. My last non-Apple computer was some ThinkPad, with vents on the bottom, and I remember always chasing some book to put under the computer to make the vents free. Boy, how I hated that.
I doubt any computer you can life will kill you if you put it on your lap. It might be uncomfortable, but it won't kill you. (I'm not about to put some of VAXen I've seen on my lap even though I can technically lift them, but...)
People do all kinds of terrible things to their body. That you do something and seem to be getting away with it doesn't mean you should. Talk to a real doctor trained in this (your regular doctors probably is not) for details.