DeathWhitch, When you hold the PS button and enter the quick settings menu, it won't allow you to set brightness at 100. If you click the slider and slide it all the way to 100%, it bounces back to 75%. You think I haven't tried that ? I only opened up a thread here because I didn't know how to, not because I felt lazy.
My guess is that some games set an auto brightness for a couple of reasons: There could possibly be some lighting and shadowing visual effects (such as the flash that treasures in Uncharted give off, or polished surfaces glimmer realistically) that may not be noticible at 100% brightness. It would also maximize your battery life. As it is, Uncharted is already down to about 3.5 hours on a fully charged battery. At 100% brightness, you'd probably be down to an hour or two.
I just bought my Vita yesterday and I too wish this automatic brightness feature would go away. I feel like it would look sooooo much better if I could run it at 100% at all times. Yea I get it about the eye strain for some, and battery life, blah blah blah... but thats what the adjustment is for... so each person can set it to where they want depending on there situation. Hopefully a future update gives us the "auto bright ON/OFF" feature!