Quite ridiculous position.
To my knowledge, there has never been a suggestion of ever not allowing Canadian citizens back into the country for any reason than in these cases.
This is not close to seeing Canadians convicted of crimes out-of-country not being allowed in. On the contrary, we have allowed Canadians convicted, sentenced and serving time for heinous crimes back into Canada to serve their time in Canadian prisons. ISIS is a one-off terrorist pheomenon we hopefully won't see again.
Going overseeas, and committing a crime is far diffrent than knowingly giving up your country, family, friends, job, lifestyle and freedoms to join an internationally-identified terrorist organization. Those that chose that twisted path should remain out of country. They said it was what they wanted.