Did someone move the cheese?

Adrian is absolutely right about the scrum master role. A good SM should of course be a "servant-leader" - a facilitator and mentor, not a manager and not someone who tells the team what to do. However, self-organizing teams do vary in their capability and maturity and very often the SM is chosen because of his or her superior experience, skills and ability to lead by example. It's natural that some personalities have a tendency to be dominant among their peers but having teams that are self-inspecting and adapting is a good way to counter that and to channel energies in the right direction.

"[...] and a Scrum Master is added to lead the team and interface with the Product Owner"

Honestly I don't have much to say about this topic but I can't stop not commenting when people claim that the Scrum master should lead the team.

If the team is self-organized then "theoretically" nobody leads it. The Scrum Master should be just a facilitator. Why so many people claim that the Scrum Master is the boss and he should lead the team when the Scrum "theory" says otherwise?

Many people advocate Agile and self-organized teams but in reality most of them want to be bosses and give commands and orders to the members of the so called self-organized (some would call it self-managed team). :)