Loco, the problem we are having in communicating is due to the fact that we are talking around each other. Your example about pulling someone over is not a good one because you are not providing enough information about the police encounter. If you are pulled over for speeding, the police can request license, registration and insurance. If the driver can't provide these documents or gives the cop expired or fake documents ( regardless of whether or not the driver is blonde haired, blue eyed and speaks perfect english) the officer now has reasonable suspicion that a crime may have been committed and therefore he is entitled to investigate further by questioning, detaining and searching the driver of the vehicle. Of course, the cop can also casually observe the interior of the car, any passengers, etc. If after a further investigation probable cause has been established that a crime was committed, the cop can arrest the driver.
Now, if the driver has all of the required identification/ documentation that every single American is required to have on them while driving a car, then that officer has not established the requisite reasonable suspicion that is required for him to determine the driver is in the country illegally and therefore he cannot force the driver to prove he is legal. Do you understand what I'm saying here?
Regarding your second point, how can an officer reasonably suspect someone of tresspassing unless the officer observes a suspect breaking into a home or climbing a fence onto private property ( or receives a call from the radio dispatcher saying the individual that they are observing matches a suspects description for tresspassing)? The answer is that he can't. As I have said already, being Mexican or Mexican looking is never enough to justify reasonable suspicion.
I'm not a cop. I passed the NY and NJ bar exams so I know a little law.