Is voter registration legal or illegal on federal property?

Indian Health Agency Barred New-Voter Drive

QUOTE
Officials at a federal program that runs hospitals and clinics serving Native Americans this summer prohibited employees from using those facilities to sign up new voters, saying that even nonpartisan voter registration was prohibited on federal property.
UNQUOTE

If voter registration is prohibited on federal property, then is it legal for the Department of Defense to conduct voter registration drives on military installations?

I can see barring organizations from conducting registration drives on federal property (Hatch Act), but making reg. cards availible for people to fill out and submit would seem to suggest that you could not register to vote at the Post Office or the DMV.

While it depends on how the registration drive was organized (backed by a partisan organization), there is no mention in the article of anything that would seem to violate the Hatch Act. Here's a link to an advisory put out by the Office of Special Counsel referring to an earlier case, and it includes a checklist of circumstances that would apply when considering barring a registration drive. Number 4 could very well apply here, but that is the most subjective of the criteria (how do you know if a seemingly non-partisan group of people want to register Blacks because they're largely Democratic or because they're largely un-registered?).

As it stands, it sounds like the Department of HHS is just trying to impede the registering of largely Democratic Native Americans to vote.