Actually, there is plenty of open dissent in political parties. I'm not sure how you conclude they do not permit dissent
Well there is and there isn't. That, like Church doctrine, depends on how close the dissent is to the core. Two examples. Govenor Bob Casey, a liberal Democrat was virtually excommunicated from the party for his opposition to its orthodox abortion stance, and was turned away from the 1992 Democratic National Convention. In a similar vein, The mainstream Republican Party doesn't want anything to do with people like Alan Keys or Steve Forbes. Far to radical and upsetting to the status quo.
One reason there probably isn't much"open dissent" of any substance in the Church proper is because only those people who believe in it and are faithful to it are active in it. Those few who openly dissent publically to a great enough degree are excommunicated, and the vast majority of those who would openly dissent, but have no desire to grandstand in public, simply fall away from the church and isolate themselves from its culture and people, and go on with there lives without public prancing. The faithful, active members are, by definition, a self selected group. If you really don't or can't accept the teachings and principles of the Church, you just leave.
A few insecure egocentrics, like the September Six, feel the need for their fifteen seconds, but most who cannot handle the requirements of the Gospel simply fade from activity without wailing and gnashing of teeth.