>is this legal? or even ethical?
If I ask a public httpd for the content of a URL and it sends it to me, it's my choice to render it how I want. The fact that I might choose to disregard various CSS and overlays and javascript delivered along with the text body content is up to me.
If somebody really wants to implement a fully effective paywall they're welcome to put their content behind a fully active content generated system that requires a unique username/password, same as signing in to an airline's mileage program/ticketing system, your personal paypal account or online banking, etc. Nobody is out there building paywall bypasses that work to render the content of somebody else's Citibank account.
I strongly suspect archive.is is buying subscriptions to the big outlets with actually hard paywalls. The fact that both OP and the Magnolia1234 fall back to archive.is for the actually challenging paywalls support that. That's the difference to a Citibank account: you can't simply buy access for that.