People who choose to remain dual nationals do so, so they can be afforded the rights of both countries and freely travel between them enjoying those rights - effectively they are hedging their bets to ensure a better deal for them personally and retain the option to live in either country at their whim.
Its quite possible to refuse your dual nationality and accept one country over the other, doing so has no impact on your own personal identity, it just effects your legal status
So whilst some may see it as unethical to deport people and revoke their secondary citizenship (which as others note has specific terms on its application), the people who are choosing to retain dual citizenship do so to retain options, so why is it so unfair that governments also retain the options to take advantage of how people choose to have their citizenship?
I understand both sides of these arguments and its very difficult not to let the nature of the crimes they were convicted of cloud my judgement on this (its absolutely abhorrent and the Rochdale issue is still on going as this is 47 girls out of a supposed 1000 abused by men in Rochdale) but I think that whilst its not entirely the best case scenario there has to be some sort of deterrent to people (who choose to have dual nationality) over breaking the oath of citizenship in any country, otherwise they would make use of their dual nationality and flee
(as one of the gang did before he was put on trial)
Now I want to add its rather disingenuous to frame this in anyway to be because they are brown, I've yet to see anyone claim that or anything remotely similar, what they have said is that people who have emigrated to a country and hold dual citizenship are open to be returned to their own country AFTER they have completed a sentence - I'd damn well hope that if we are doing such, that if the perpetrator of a serious/heinous crime is a dual British/American, British/Swiss, British/South African, British/Taiwanese that they too would have the option open to them to be deported to their respective country of Origin. This isn't about colour, if you want to claim anything claim Xenophobia, don't say its race, I think Brexit has proven its more Xenophobia than anything else