the Society has put the kibosh on dubs taking these kinds of surveys.
True, but....
1) that restriction was stated in a letter read to the congregations 2 or 3 years ago. It has not even been mentioned since. How many people remember that the letter was even read? 75% of JWs couldn't tell you what the theme of the Public Talk was 5 minutes after the Watchtower study.
2) Given the anonymity of answering questions over the phone, perhaps a reasonably large number figured "hey the elders will never find out how I personally answer these questions."
The proprtion of JW responses (a couple hundred) to the total sample size (35,000) is on the same scale as the number of JWs in the US to the total population (1 milllion out of 300 million).
The only thing that might skew the results are people who consider themselves JWs, but really aren't. In one of my previous congregations we'd run into people in field service all the time who said "oh I'm a JW too", but really it was a case where they studied 20 years ago, never got baptized, but figured they had somehow "joined". But I don't think there would be a significant amount of people like that anyway.