Let's get one thing out of the way: In a perfect world, all LEOs would always know all about the laws they enforce and would always be polite and courteous as they did so.
This is not a perfect world. Odds are, however, that the LEO of whatever sort knows a lot more about what the law actually is than does the average citizen.
A direct quote from the Chief of the Marfa Sector of the Border Patrol when asked about rudeness and discourtesy: "They shouldn't be." And, then, "Human nature. Maybe the agent is having a bad day...Are they instructed to be polite, professional, courteous? Yes...If you run into agents that are not courteous and professional, let us know."
You should know that the Border Patrol is now additionally charged with anti-terror and anti-drug work as well as the original checking for citizenship. Expanded legal powers.
Checkpoints are located at chokepoints for travel. Very logical. No place to run to, for a Bad Guy. No place to readily make a U-turn and then turn off onto some side road or street.
One thing to remember at these checkpoints is that many of the people there are newbies, trainees. They're nervous and unsure. That tends to make them somewhat overzealous, hoping to please those who observe. But it's a lower-pressure way for them to break-in, to gain experience.
I've lived down here next to BBNP for 25 years. I go through the Cathedral Mountain checkpoint every time I go to Alpine. Maybe 25 times a year; even more, some years. I go to Ojinaga, Mexico, for shopping; maybe monthly.
I've seen officiousness and foolishness, but I've yet to be treated rudely.
I roll up, lights dimmed or off. Sun visor up. Window down. I try to speak first, with a grin and, "Hey, how's your day going?" Or, "Don't you just love having an outside job?" Invariably it throws them a bit off stride, and they commonly smile, MAYBE ask if I'm a citizen, and wave me on. If my truck is muddy, they might ask if I live in Terlingua, and grin at a, "Yeah , 25 years."
Last time I came through the Del Rio checkpoint (near Comstock), there were two BP guys standing there, just BSing. As I rolled up I could see that both were Latins. As I stopped, I heard the tail end of the conversation; it was in Spanish. So, me being me, I said, "No es verdad!" They both cracked up laughing, and without another word waved me on.
Generally, you get what you ask for.
Art