If you're a Belgian citizen, and move to, say, Canada, you'd file taxes in Canada and that'd be the end of it. If I, as a US citizen,move to Canada, I'd have to file and pay Canadian taxes, and file - and maybe pay - US taxes in addition.

What you're talking about is if, living in Belgium, you have income from other countries that needs to be reported, which seems fair enough - after all, you live and pay taxes in Belgium, not those other countries.