It's treated special cause most shells handle undefined variables as empty strings so `rm -rf "${base_path}/${sub_dir}"` can turn into `rm -rf '/'` and users commonly don't expect that.

While that case may be simple to catch the writers of gnu rm also recognize that scripts tend to not be well tested and decided "better than it currently is" is better than "we didn't do any mitigations to a common problem cause the solution wasn't perfect".