Why can't the owners of MLS raise the salary cap? Sure MLS help soccer in the United States but I wouldn't go as far as to say it helped the USMNT a lot. I understand that most of the USMNT members are current or former MLS'ers but the majority of the starters play in Europe. If MLS wants to capitalize on the American's run during the World Cup (they could get to the semi's with how easy of a route they have to get there), then they need to raise the level of play with increasing the salary cap.
What you're saying is logical....but owners like Kraft push cheapness upon other owners. I personally don't mind some players transfer out of MLS, but only if need increase salary cap with some transfer money and DP don't much space in salary cap. like each DP take only 100k of salary cap.