Normally this is true, but the combination of highway classification and access tagging is flexible enough to accommodate unusual situations. Some regions have stricter norms than others. Some regions have traffic signs that conveniently support these norms without requiring mappers to make a judgment call, but not all regions are so lucky.
iD’s support for access tagging is still incomplete. The scheme is much more complex than the existing field lets on, and there isn’t very much validation about it yet. It’s a known issue: one of this year’s suggested Summer of Code proposals is specifically about building out this feature, though it’s too early to say whether we can expect anything to come out of it this year.