In baseball, younger players have little leverage in contract negotiations. He feels cheated and the only leverage a young player has is making the team that played hard ball suffer down the line, or to at least make that team think there is a real chance that how they treat him now will impact that decision.
He feels cheated? The Sox awarded Papelbon the richest contract in history for where he is in the majors at his position; first year of arbitration eligibility. This happened in January of this year. The one year contract was over six million and more than an 800% raise from his prior contract.
According to this...
http://www.boston.com/sports/baseball/redsox/extras/extra_bases/2009/06/papelbon_strong.htmlhe's happy to be in Boston, and doesn't sound like someone that feels "cheated".