The ad falls under free speech. What people need to understand, but keep forgetting, is that you don't have to like every bit of free speech out there, and chances are there will be plenty you don't like, but the opposing views have as much right to comment, as do you and those who share your views. In America, just because someone doesn't like a particular comment, doesn't give them the right to shut up the commenter. Read the Constitution. It makes it very clear. And yes, sometimes people will use rather strong language to express themselves, especially when they feel strongly about a topic.
There are common sense exceptions to free speech. You are not allowed to slander or libel people and you are not allowed to incite people to violence, or to create a panic, like shouting fire in a crowded place when there is no fire, because people can get trampled to death trying to get away from a non existing fire. But for the most part you have the right to say what you want.
Political and religious speech can generate a lot of heat, often a lot of strong language, and often some nasty allegations. It is up to the person being accused to decide whether to challenge the accusations, or to let it go, or to admit them. Our country was built on political and religious freedom. Trying to shut up opposing views is illegal, and undermines the ideals that this country was founded on.
Don't worry about the strongly worded ad on the bus. It is Obama's place to decide how he wants to respond to it, or even if he wants to address the issue at all.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau