Because the local transit authority actually has a sense of humor:
Commentary from dcist.com:
The subject of this latest comedy transit video is, of course, Inauguration Day, though we actually think Metro could run parts of this one year-round, since it includes sage advice about standing on the right and staying clear of closing doors. It features "appearances" by cutouts of President-elect Barack Obama, Vice President-elect Joe Biden, President George W. Bush, Vice President Dick Cheney, First Lady Laura Bush, Secretary of State Condoleezza Rice, and, as though thought up during a late night hash smoking session, an inflated balloon of the figure from Munch's The Scream painting as a stand-in for clueless out of town passengers. Don't miss the little balloon getting squished between some Metrorail doors! It's fun for the whole family.
Also, is Metro insinuating that Joe Biden doesn't know to walk left, stand right? Say it ain't so, Joe.
dcist.com also has links to earlier, equally hilarious Metro videos.