In point of fact, they can hold you or prevent your re-entry to the US for pretty much any reason, because you are not yet legally _in_ the US, just like any mail or phone conversations crossing the border can be snooped on at will, and your person and your luggage are subject to (warrentless) search.
You are correct!
My niece and I did a run over the bridge to Canada in Detroit to buy some candy (truly, Canadian Chocolate is yummy). We decided to take the car that she had borrowed from a friend who sells cell phones. We were unaware that he had his inventory stored in the trunk.
We went to Zellers and bought about 50.00 worth of candy for her to take back to Cleveland, got gas and headed back. The border patrol asked if we had anything to declare. We said chocolate. The guy looked at us like we were criminals. (who drives across the border with Ohio licence plates to buy chocolate???) He asked us to pop the trunk. WELL! we were detained for about 2 hours while they tried to contact the owner of the car. Finally, he verified that he worked for this company and it was his car. He had to call HIS boss to verify the fact as well.
I should have taken my soccermom van and we would have been fine.
Hmmm...nice point. I did not know that.