It is diplomatic protocol for the host country to allow any representatives from the country being honored who wish to attend. The former Soviet satellite states of Eastern Europe bend over backwards to be polite and accommodating to any Americans, especially those there on official business. The Hungarians are second only to the Poles in hospitality. The Czechs are a close third. Zer0's boys weren't there because he didn't want them to be there.
Notice SOS Clinton will be there a week later. The Hungarians would have delayed the statue unveiling to accommodate her schedule had she wanted to be there. Clearly, she did not.