Cause of course SOME OF THEM are migrants, but most of them have been French for generation, they only have the French nationality, saying "send them back to their country" just means send they back to France cause they were born in France, their parents were born in France and so were their great parents...
Most of them may have been born in France, but they don't feel that France is their home country. I have even seen third generation immigrants supporting Algeria and Tunisia during the football matches against the French team. Similar to the situation in USA, where third generation Hispanics support Mexico in matches against the US.