Well in our case paste wasn't disabled. It was definitely a case of a PM coming up with a quick and cheap 'fix' to the problem of getting so many people typing

Well in our case paste wasn't disabled. It was definitely a case of a PM coming up with a quick and cheap 'fix' to the problem of getting so many people typing their email address wrong the first time. We (from the tech side) put together a bunch of options, including validating the domain and giving the user feedback if delivery doesn't succeed immediately. But it was put on a backlog of 'nice things to implement' and never delivered.