The DOT, just like any other government agency, only does what they need to. If their efforts have to go either towards testing the headlights of new and upcoming cars, or to testing some random xenon bulbs, they'd have to appropriate their funding for what is more necessary.
The light output of 6000k xenons is on average about 3/4 that of a 4xxxk set, which translates to lower contrast and therefore a lesser ability to spot objects with slight differences in color to their surroundings (not to mention that the light does not reach as far).
Borrowing from an article I read about HID headlights, it has been shown that the bluer the hue of a particular light is (the higher the color temperature and wave energy), the greater its "dazzle" effect is ("dazzle" is perceived brightness of the light based on how much it impairs your ability to see other things in the dark). This is why the light "seems" brighter.
I don't think they deliberately turned down the light output of the 6000k's so as not to completely blind oncoming drivers. Given that you're putting the same amount of electrical energy into the bulbs, the extra energy that would have gone into producing that extra 1/4 of the light output is instead being put into the photons themselves to make them higher energy (and hence the higher color temperature).
As far as offroad use goes, it doesn't literally mean offroading as in 4x4's. "Offroad use" just means off of any public road that is under DOT regulation. If you happen to own a really big ranch with a racetrack or highway network, you are free to use any bulbs you like, as long as you don't stray out onto public roads.