How is that this so? Most car windows are coated with silicon or other substrates that will make vinyl adhesion short lived. And because vehicles are in contact with direct sunlight and heat for too long periods, vinyl decals, which are expected to last a minimum of three years, may well not continue to. Additionally, the majority of the tints that coat car windows are also not vinyl friendly. While the vinyl decals stick longer about them, they be a problem for taking off without running the potential risk of destroying the film.
It will also be prudent to get rid of dark-colored graphics, simply because they respond adversely to sunlight because they absorb greater amounts of heat. If left for a long time of your time, the glass might eventually explode. On the flip side, vinyl window graphics adhesion does not operate in too cold weathers, particularly those under 40 degrees Fahrenheit.
This is the reason most vehicle graphics installers discourage against placing decals on windows. In addition to the incompatibility in the window coatings with vinyl, they also require much more work to install. For instance, while car bodies are okay with calendared vinyl, glass windows respond better to perforated cast vinyl, which are relatively pricey. Also, because car windows still need to serve their purpose, that is, for people inside so that you can see what's outside, special materials have to be used.
This will not say, though, that vinyl graphics will not be added to glass windows. It really serves as a reminder that installing window graphics is a more tedious and costly process. However, in the event you really will need to have graphics on your own car window, then you certainly should bring the job towards the professionals, being safe.