You may have thought you were saving your details with that shop (or not realised at all of course) but yes this is a recentish feature I think, at least I haven't noticed it for long, branded 'Shop pay' iirc.
As for legality in the EU/UK, it's just like everything else, on some level they technically asked for consent and you gave it, but yes, dark patterns abound.
Right, but the enforcement on this stuff is terrible, you probably also started receiving mailing list spam that you didn't knowingly opt in to, and nothing's going to change or come of it even if you do report it to the ICO.
As for legality in the EU/UK, it's just like everything else, on some level they technically asked for consent and you gave it, but yes, dark patterns abound.