This speaks to a purely practical problem. The anglophones who want to send their kids to an immersion program want their kids to have an additional tool, meaning for them to learn another language. But the goal is not to assimilate their children into French culture. We're not talking about francophone schools where French is the mother tongue, but rather schools where the majority of students don't speak French and so the curriculum is designed to help students learn the language. In this case, the francophone parents are saying that they want a school where they could develop as a linguistic community.
In a place like Caraquet or even Bathurst, if 10% of the students in a francophone school are anglophone children, there are no problems with regard to teaching them. These students will assimilate into the francophone group. However, if we try to develop a francophone identity project in a French school in Calgary, for example where 90% of the students speak English as their mother tongue, this wouldn't work. That kind of project could be done in Chicoutimi or Caraquet, but not in Toronto. Those people wouldn't be interested, since their goal is not to develop a francophone identity but rather to learn a language as an additional tool, to broaden their horizon on the world.