A principle tennet the the United States is founded on is that people have the right to be "offensive." The bill of rights is intended to protect the rights of the minority, not the rights of the majority, because without it Democracy degenerates into what was described in "The Mouse That Roared"as "the majority forcing it's will upon the minority."
There are those that argue that freedom of speech is inteded to protect "political speech," well in a Democracy ALL speech is "political," especially if politicians are talking about it. And the fact that "Cop Killer" is considered "objectionable" is exactly why it deserves to be protected. I would rather see the government ban the singing of "Gilligan's Island" than "Cop Killer."
Dan