The fact is that they are followers of an imam. As long as the imam is in the state of prayer and he himself is not released, those who follow him cannot be released. So the imam does not stand up and start doing other things after saying one salam.
He is relieved of his responsibility after saying the second salam, not before. So when he is free to move about, only then the people behind are free to do what they want to do. I mean, they can begin doing things individually after the imam has released them. And while imam himself is not free before the second salam, how can he release the muqtadi?
The philosophy is you must follow the imam, even in mistakes. That is the greatest lesson in discipline that the holy prayer gives for all the generations of Islam which were before or are to come. An excellent lesson in discipline and in obedience.
