On my site, I've got three different content types that form the overall "content profile": Profile, Education, and Background. Profile and Education are included in the user sign-up process. Taxonomy classification is enabled for Profile but not for Education. However, after users have signed up, both Profile and Education have the same taxonomy term applied. Meanwhile, Background content added later, which also does not have taxonomy enabled, ends up with no such taxonomy term attached.

I thus conclude that all profile components included in the user sign-up process end up with taxonomy terms applied regardless of if the taxonomy was enabled. This appears to be a bug.