Everyone in the joint had just watched last week's Frontline.
Hmm, it is an elected position and thus subject to public scrutiny. If it is ok to cheer the POTUS, then it is ok to boo him/her. They come into that position, not by divine right but through the will of the people (ok, call me ol' fashioned). Definitely a free speech issue and I would boo him for the simple fact that he was at a baseball game and he ruined my beloved Rangers now so many years ago.
Then again, the Nats suck and there is no parking around that stadium. You either park three miles away at RFK or take the Metro which has added no additional trains to alleviate the traffic during games and rush hour. Also, it looks like Baghdad around that stadium. All the construction without the sniper fire and IEDs. Hey D.C., how are your schools working out for you?