IMHO, the decision of whether to show auto-complete should be with the user and not with the website. When I install an auto-complete add-on or activate a browser feature, I expect the AC to be available on ALL input fields, whether the site owner thought that would be a good idea or not.
Now, there is a valid question on how the user should be able to configure the AC behavior, and how the website may help inform this configuration, but the decision should be with the user. The website should not have the final say.
So I would see this as more of a shortcoming of the HTML Spec.