I got one of those letters a while back. I wrote a very nice letter back explaining some of the reasons why I couldn't trust JW's to help me understand the Bible. :P I also said I'd be glad to have a Bible study with them as long as we didn't use WTS publications. For some reason, I never heard back. You'd think they would jump at the opportunity to start a Bible study.
Obviously, though, it's a scam for hours. With computers, one could crank out hundreds of generic letters personalized to each recipient in a short time, but this letter was hand written. It takes a lot more field service hours when you hand-write your letters instead of using the computer. ;)