The short answer is that some people want their car to stand out from everyone else who has the same car. Some people buy quality items and other people buy cheap items like decals. To each his own...
http://www.amazon.com/b?ie=UTF8&node=268879011
I don't know how many people actually 'tint' their headlights. Perhaps what you are seeing are the 'colored' bulbs. Some give off a yellow, blue, green, etc... color. But, those look normal 'clear' when parked.
I know a bunch of VW guys (a lot of MkII owners) who spray a light coat of red over their taillights in order to make them look uniformly red. Once a reverse or signal light comes on, it is obvious that it is white or yellow. IOW, the light coat of red paint only looks really red when the reverse or turn signals are off.
I do wonder what happens to the reflectivity of the lens with that layer of paint on the taillight. If your car stalls and is left in the middle of the road in the middle of the night, your taillights still reflect an approaching cars headlights, so the car isn't invisible. I don't know how much that is affected with a layer of red spray paint on it.