That's a ridiculous way of thinking about it. Anyone who says they don't want illegal immigrants isn't objecting simply because of the legal status, they think the process should be upheld which prevented those people from immigrating legally.
It's the same reason we don't seek to improve crime rates by legalising theft. Sure, there'd be fewer people labelled as "criminal", but the original problem would remain (and in all probability would become worse).
And why the process of immigrating legally is more than buying a ticket and a security check at the border?
Obviously, it is the reason that matters and the reason is not benign.
People want other people to like people in certain way they and then castrate Alan Turing for illegal love. People want to have slaves then they have illegally free slaves problem.
Why pretend that this is about upholding the law?