The reason that they can be returned is that legally at a Port of Entry the person is not IN Canada but once a person is admitted or physically crosses the border even illegally they can then only be returned to the country of citizenship or legal residence unless another country is willing to accept them.  

This is also the case with other countries for persons arriving at international airports or on vessels from countries other than the USA unless they claim Convention refugee status.  The transportation companies are required to carry them back at their expense if served a notice.  These procedures are set out in the IRPA and regulations.

The Canadian government exploits the same POE "IN Canada" loophole under the Safe Third Country designation of the US so any Convention refugee claimant coming from the US can be referred back.

The reports I have seen make it clear the persons entering Canada do not have legal status in the US.