I believe you are correct that there are no limits to what U.S. Customs can do so long as you are being processed within the physical borders of the United Stat

I believe you are correct that there are no limits to what U.S. Customs can do so long as you are being processed within the physical borders of the United States. At a land crossing, once you attempt to gain entry you cannot simply decide "it's not worth it" anymore and return back to the originating country. You have no rights, not even the right to turn around and leave. In an airport setting (most mid-sized and larger Canadian airports have US Customs processing stations before you depart), you can indeed decide to turn around and not proceed with the crossing. From a legal standpoint I believe it has to do with the fact that at a land crossing you are already on U.S. soil by the time you reach Customs. In the airport scenario above you are subject to the laws of the originating country since you haven't left their territory. It goes without saying that refusing to proceed with a crossing at an airport once it has commenced would likely have other very serious implications. The local authorities would almost certainly want to have a word with you and you'd be flagged by U.S. Customs as suspicious.