"Non-consensually", as if you had to ask for permission to perform a GET request to an open HTTP server.
Yes, I know about weev. That was a travesty.
robots.txt is a polite request to please not scrape these pages because it's probably not going to be productive. It was never meant to be a binding agreement, otherwise there would be a stricter protocol around it.
It's kind of like leaving a note for the deliveryman saying please don't leave packages on the porch. It's fine for low stakes situations, but if package security is of utmost importance to you, you should arrange to get it certified or to pick it up at the delivery center. Likewise if enforcing a rule of no scraping is of utmost importance you need to require an API token or some other form of authentication before you serve the pages.