It is simply about upholding law. 

Canadian Prime Minister Justin Trudeau worded it perfectly, in regards to their border, and unlawful 'migrants' migrating there, I think.

"Canada is an opening and welcoming society, but let me be clear. We are also a country of laws" Trudeau said..."There are rigorous immigration and customs rules that will be followed. Make no mistake."
http://www.dailywire.com/news/20015/canadian-pm-justin-trudeau-would-prefer-you-not-emily-zanotti#exit-modal

In my opinion, that was our mistake years before-failing to enforce our very good immigration rules and laws.  We welcome immigrants as we should of course, but until today, we failed to legally address all who are illegally here.  So here we are now.