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I think the logic is that there is a tipping point where you're not just talking about someone moving to the Costa d sol, but actually leaving your country to become part of a directly opposing state, that's counter to absolutely everything you believe in and has the explicit aim of destroying you...and then tries to destroy you.
Obviously it's a bit different in Oz where its mainly just drinking and beachwear that's diametrically opposed.
It's amazing... Australia trying to do a similar thing with nationals who've gone to the middle east to join Isis, you can't just strip someone's citizenship because you don't agree with them, no matter how fucking crazy they are they're still Australian...