Because with RFC 3161 it's possible that exist two compliant systems
which can't interoperate properly in some situations because one
accepts that
PKIFailueInfo contains more than one bit with value one (1) and the
other not!
It seems that the text could say 'MAY only be any of the following
values'.
as the list is a restriction (and extension) of the values define in CMP.
Or: 'Only the following values MAY occur'.
I could detect an invalid hash algorithm and an unsupported extension,
an unacceptable policy, and even time source not available all together.