Is there a posix compliant 'shell' we can try? As above, bash and sh flunk out the same way. If there's no other shell that passes this test then they should change it.
And if you think about it a comment is a comment ... there is no active code in a comment line so parsing the backslash is dumb. You might have commented out some line of code that has a trailing backslash and you want it *inactive* not killing the next line of code too by trying to make it part of the comment. Dumb!