Earlier quoted context omitted.
A million people probably have at least 10x more relatives and friends on the outside. Ways of breaching the great firewall are well known even within China. Shouldn't there be...numerous reports of missing persons if this is the case?
That's what I was thinking too. Westerners visit Xinjiang province with cameras. If 1m people disappeared, I'm sure someone would come up to a Westerner to tell their story. 40,000 people were disappeared en masse in Chile by Pinochet. That story made it out very quickly, and only a few photographs exist of it
China isn't a western country, and I don't think you can count on being granted western-levels of freedom of movement there, especially in "sensitive" areas. For instance, I understand you need a special permit to visit Tibet, which is another province with similar ethnic tensions.
The level of surveillance in Xinjiang is so intense that the locals will often get questioned by the police about contacts with foreigners (e.g. getting a phone or skype call from abroad). The locals probably aren't eager to talk to foreigners about sensitive topics because they fear reprisal.
China also has special visas for journalists, and something tells me that that they'll deny the application if your itinerary includes areas where they don't want people poking around. If you're applying for a tourist visa, in some cases (like if you work for a newspaper) it's advised that you promise that you'll not engage in journalism during your trip. I'm guessing the Chinese will interpret a bunch of foreigners poking around sensitive areas with cameras as people practicing unauthorized journalism, put a stop to it, and maybe even deport those people for visa violations.