The US is most certainly allowed to enforce their border and from what we've been hearing about up here, since Jan 20th, very vigorously. Here's an example:
http://www.cbc.ca/news/canada/montreal/canadian-denied-entry-us-immigrant-visa-1.4011202
Whether it's accurate, or not, there is a rapidly forming sense that if a Canadian wants to visit the US, they had better not be sporting an excessive amount of melanin. And, it wouldn't hurt to adhere to the Judeo-Christian mindset...
There has also been a significant increase in the number of refugees in the US crossing into Canada because they are afraid that they will be put on a flight to their home countries. They are risking life and limb to cross the border at unofficial crossings in remote areas in the depth of winter in order to get a chance to claim asylum. Because of the "safe third country" agreement, if they show up at official crossings, they will be turned back because they have already claimed asylum in the US. So far, they've been generally greeted with sympathy. If the stream turns into a flood when the weather improves this spring, who knows how that might change?