What this screws over is there was plenty of people from US visa waiver countries who decided K-1 was too hard and just flew over to US and got married. They would then apply for Adjustment of Status. That is big door being shut close because B-1 is non immigrant intent visa.
My room mate from college did this with UK foreign exchange student 20 years ago. She came over on visitor visa, got married and they got a lawyer to fix it all up.