Why can politicians (and anyone else) use copyrighted music without the artist's permission?

There have been numerous allegations over the past weeks by musicians against the McCain campaign based on its use of songs in its advertisements and campaign rallies without the prior authorization of the artists who recorded those songs. Some of these artists include Jackson Browne (who sued the McCain campaign for the usage), Van Halen, Heart and the Foo Fighters. In all fairness, Obama was asked by Sam Moore to stop using his song, "Soul Man" as well.

Many of the artists feel that they should have the right to keep the McCain campaign from using their songs because they feel that (not only should they have been asked) it appears to other people that they have not only sanctioned the use of the song, but they have endorsed the candidate by allowing the song to be used.

So, why hasn't McCain stopped using the songs. It's actually very simple. The McCain campaign claims that they've purchased blanket licenses from ASCAP/BMI. A blanket license (often purchased by stadiums, sporting arenas and the like) gives the purchaser the right to use any of the over 4 million songs in the ASCAP/BMI library. They have the right to use them at campaign events, in advertisements or even broadcast them.