I am not a lawyer (and also Canadian) so I am not an authority on this subject, but my understanding is that legally the Border (or perhaps a "border zone") essentially exists within 100 miles of the actual physical Border between Canada and the U.S. and the U.S. and Mexico. Within that space, Border Patrol officers are allowed to stop and question you without a reason. For example, the U.S. border patrol has numerous checkpoints in Texas within 100 miles of the Mexican border. I have been frequently stopped while traveling in southern Texas, asked my nationality and to present my passport for inspection.

The first time this happened it struck me as odd because I had already been legally admitted to the U.S. so I checked with a lawyer in Texas. He explained to me that everyone is legally obligated to answer any legitimate question the border patrol asks about citizenship within 100 miles of the border and you can be arrested if you refuse.

Beyond questions about your citizenship, you can absolutely refuse to answer and of course they cannot search you or your vehicle without consent or probable cause (presumably "thinking you might be a terrorist" is not yet considered probable cause).

I can understand the frustration of residents in Washington state, but in Texas I have personally witnessed the border patrol catching illegal immigrants and in one case intercepting drugs (smuggled on an intercity bus) so this does appear to be a legal, legitimate and effective law enforcement practice, albeit a very annoying and inconvenient one.