Senator Rand Paul pushes to “remove a federal ban on guns in post offices”

Given that permitted concealed handguns are allowed virtually everywhere (Fedex and UPS stores, banks, etc), what is the argument for why post offices are so different?  From the Washington Post:

Leaders of the Senate Homeland Security and Governmental Affairs Committee were scheduled to approve a wide-ranging postal reform bill. . . . 

The committee tabled its deliberations at midday. Aides said the Baldwin amendment, along with another proposal from Sen. Rand Paul (R-Ky.) to remove a federal ban on guns in post offices, would require more study. . . .