I am not American, so correct me if I'm wrong, but isn't the melting pot metaphor about taking people from different cultures and backgrounds and "melting" them together into one much more homogenous culture?
The American melting pot rejects plenty of aspects from other culture, and does more along the lines of "this is how we do things in USA, take it or leave it" rather than assimilation (at least from what I've observed in the recent decade). From elementary schools to the workplace environment, things have only been getting harder for immigrants, to the point where I'm not sure if USA can claim to be multicultural by strict terms.
By contrast, a mosaic setup like in Canada, where every cultural aspect brought into the country are welcomed and celebrated, is much more comforting to immigrants. Because multiculturalism is actually incorporated into the Canadian federal policy thanks to, surprise surprise, the previous Trudeau.
The point of moving to a country is to make a better life for yourself. We are not trying to comfort immigrants, we are trying to make countries stronger.
There are plenty of abhorrent views held by people out there in the world. What part of the mosaic should that be part of?