I think I know were the confusion is att.
There are inland check points from CBP and they simply ask if you are a Citizen and some folks get all bent out of shape at this, there are a number of these check points arround the County of San Diego and the easiest thing to do is roll the window down and when asked just answer "yes sir" or "mam" and off you go.
Crossing an Intetnational border is a totally different story, at the POE the Immigration and Customs agents are tasked with the security on the entiere nation, it used to be more relaxed and most persons were honest, and yes profiling was used, (little secret "it still works") today there are a lot bigger threats by very nasty persons and nations, so if today they require a document at the border, it is easier to have one.
At the inland check points no you do not need a passport, it now becomes a Constitutional right of freedom of passage, at the border you are entering the country, not travelling though it, so freedom of passage can be restricted and documentation requiered, we live in a very different world after 9-11.
navegator