NEA2 - The Safe Third Country Agreement means no such thing.  It was a an exploitation of a fictional legal loophole by the government to get around the 1985 Singh decision that required every refugee claimant to receive a hearing before an independent decision maker/tribunal before being refused.  The SCOC ruled that every refugee claimant "in Canada" must be accorded fundamental justice (our version of due process).  

The legal fiction is that persons who are being examined at a Port of Entry are not technically in Canada so they can be sent back to the USA.  However if they manage to cross the border then they are in Canada and must be processed.  

It should also be noted that the Immigration and Refugee Protection Act (IRPA) and prior immigration Acts in implementing our legal international obligations dating back the 1951 Refugee Convention provides that refugees are exempt from prosecution for illegal entry and use of false documents at section 133.