You got me wondering what's going on then. If you look at AFC's FAQ page it lists some Qualities of the MC cable types
"The difference between HCF and MC cables (as manufactured by AFC Cable Systems) is the grounding capabilities of the cable assemblies.
Type HCF cable has a green grounding conductor plus the armor/bond wire combination, thereby providing redundant or isolated grounding capability. HCF is a Type AC cable and is manufactured in accordance with U.L. 4.
HCF cable may be installed in compliance with NEC 517 for health care facilities.
Type MC has only one equipment grounding means, a green grounding conductor. There are types of MC cable manufactured by AFC with two grounding conductors for additional grounding capability."
This is from page: http://www.afcweb.com./faq.html
and would seem to indicate that it is not approved for Grounding. I can see why you're questioning it.