Inconsistent?

Articles like this one portray Iraqi election boycotters as saying that elections cannot be legitimate if they are held under occupation. Today, however, Spencer Ackerman notes that key Sunni leaders will still agree to participate in writing the Constitution. Why is it illegimate for Sunnis to run and vote in an election, but not for their unelected leaders to represent them in writing the Constitution? And wouldn't a public debate over the constitutional provisions be far more susceptible to foreign pressure than an election by secret ballot?