The common sense aspect of this is typically lost on progressives.

Is there a visible and measurable difference in attitudes in immigrant families, if you compare the 1st, 2nd and 3rd generations?  Of course there is. Whether we want to formalize this as a hard test for adherence to the basic freedoms and beliefs in our culture or not, acculturation happens, and every immigrant family experiences it. Whether you measure it with adherence to the old religion, old modes of dress, old political preferences, etc. they all fade and get replaced by Canadian versions. Quick, which generation has more hockey players?  Is more accepting of gay marriage? You'd have to be a total goofball not to guess correctly.

All screening would do is improve the rate of adoption, and bring in people that might get up to speed in that 1st generation, rather than the 3rd.