It could have to do with the song that was played just before to keep the pace going. I mean, this is one of those songs that are played by many artists in al kind of speeds. Suppose this is played after raise your hands it will probably get played a lot faster than when it is played after someday i ll be saturday night.
Also, this is the band starting the song, so jon needs to sing it in the tempo the band plays it. Once it starts you can't suddenly start singing it much faster or people would go like... : huh? Whats wrong?