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.
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.