“Why are non-citizens voting at all?”
Because we do nothing to prevent them from voting. Our politicians don’t care, they are falling all over themselves to give amnesty to these lawbreaking citizens of foreign countries.
They are voting because they are being told to (and who/what for) by their SEIU or 'immigrant advocate' handlers. There's some evidence that the majority of Illegals and green card holders are unaware they are committing (another) crime. Not so with their handlers. Bust that link in the chain.