As already mentioned, this is a status-by-design.
This has also been raised on Meta.SO, where the answer points to a blog entry by Jeff Attwood called New Per-Site Metas; an introduction to per-site metas across the entire network (July 2010):
Reputation is always inherited from the parent site. You cannot gain or lose reputation* on the per-site meta. This also means that some reputation related functions like the rep graph and bounties are not enabled on the per-site meta.
* however, there is one exception: extreme misbehavior on the meta site will affect your parent site reputation. And not in, shall we say, the “good” way.
Meta.SO is considered different (and separate) from SO since it actually addresses network-wide meta-related content, rather that SO-related/specific content.
Having said all this, it is still possible to obtain reputation-related badges on per-site metas. For example, here's a list of Mortarboard badge earners to date:
I think the motivation behind this is that, eventually, there may be a possibility to switch per-meta sites over to full-fledges sites.