Regardless of political affiliation, or ex-players' beefs with Ravens management, or commitment to new teams' practice schedules, there is absolutely nothing in the players' contracts which require them to attend this event. In particular, this applies to former Ravens players. If anything, Birk and the others who fulfilled other responsibilities should be commended for standing on their principles, or not being two-faced, or adhering to new contracts.
This White House visit is nothing but a political photo op for whoever is in the White House, plus some PR for the NFL. Frankly, it's a waste of time for the Chief Executive, who should be focused on fixing the economy than yacking it up with big boys half his age. Just like one of our former Presidents who gave up the President's Yacht, someone in the White House should end this annual folly.