The reason they've chosen a federally owned building is the same reason Jared Taylor held his last two American Renaissance conferences in a state-owned facility in Tennessee—the First Amendment. In a government facility, restrictions on speech have to be "content-neutral", i. e. not based on your opinions.
I'll note that Richard Spencer has recently been honored by an attack in Salon Magazine (haven't we all?): The Hatemonger Next Door, Lauren M. Fox, September 29, 2013. He emailed us to say it was a "strangely flattering hit-piece."
He's responded to it here.