This is the best analogy I can give you with regards to tire pressure. A tire is like a basketball. The more you fill them the more they will bounce (if you've every played basketball you know an overfilled ball bounces more, and as such, is far more difficult to control than a properly inflated one). The same principal holds true for a tire. That bouncing is, however, controlled by the shocks and to a degree the springs. On cars that come with the new eco tires (specifically designed for higher pressures and less rolling resistance) will also have shock valving that has been recalibrated to better control them. Unless you have a heavy duty shock or preferably one that is rebound adjustable, significant increases in tire PSI can take a toll on stock suspension components.
The "car" manufacturers recommended PSI is based on a lot of testing. It is a compromise based on car weight, maximum load, desirable handing traits (performance car vs family sedan for example), ride quality, economy, safety, etc. The "tire" manufacturers rating is simply the maximum cold temp PSI that will maintain the maximum load rating of the tire. It does not address any of the "car" manufactures qualitative requirements. The engineers from both know a thing or two more about this process than we do and neither of their recommendations should be taken lightly. The car manufacturers numbers while not necessarily cast in stone, can be adjusted in small incremental changes to "refine" one or more of the intended design perimeters previously mentioned.
