I've spent time arguing with some fairly large and supposedly security-minded entities, about just this practice -- delivering login forms over HTTP. It doesn't matter that the specified destination is HTTPS. A MITM can do whatever they want to the form and its enclosing page while it's on its way to the client browser.
There's a quite successful startup, much beloved by HN, that has exactly this problem with one version of their sign on page. I won't say which one, because I don't want to make targeting too easy. I've been having a back and forth on a support ticket with them, after noticing the problem. They did say they followed up and looked into my concern. But they compared their design to Facebook et al. and said they were following "best practices".
An aside: Once you start using the term "best practices", you need to take a serious look at your design and engineering perspective. Especially in security, your thinking should always be challenging the established model. You may not find a better way, and you should be very skeptical of yourself when you think you have. But you need to be a bit... paranoid, always challenging the "established truths".
EDIT: To that company, if you happen to read this. I posted this after emailing you. And before the coffee fully kicked in. It wasn't my intention to create so blatant a cross reference for you to follow between me here and me in email. (I.e. not some twisted version of karma; just trying to make the point.)