The topic here is how the signup form is submitted.
Which protocol the form is submitted via is only half the security issue. The form itself needs to be served over https also to avoid a MITM attack on the destination of the form submission.
In theory if you're browsing a site over HTTP, the initial request to the form will be HTTP, and then you'll be 301'd to the secure signup form. A check could happen before the 301 is issued (instead displaying a warning message). That may not be how it is done on any given site, but my point is that it is possible to handle gracefully.
But yes, clicking an https link directly would cause issues.
Which protocol the form is submitted via is only half the security issue. The form itself needs to be served over https also to avoid a MITM attack on the destination of the form submission.