Tinkerbelle: Great question... one that I have asked to 3 separate pioneers, without a good answer from the Bible.
In fact, reporting your preaching time, seems to directly contradict Jesus' words at Matthew 6:1-6, where he warns them NOT to report their good works to men.
The best response I got, came from a pioneer on my doorstep: that they DON'T get recognition from men... but that they had to keep track of their ministry efforts, and make sure they were preaching to every corner of the world. (Can't GOD keep track of those details?) I can understand that they might need to keep track of how many magazines they need to print, but not why you have to write YOUR name on the report card. And even though they "don't" get recognized by men, they still have a requirement of 70 hours per month, for her to maintain her position as a pioneer.
What would happen, if a pioneer didn't turn in her field service report?
It's all about thought control, keeping them believing that their work is serving God... but they really just want to ensure that you are only having approved conversations with approved JW followers. It's no secret that most of their time is NOT spent preaching. They waste hours, driving from place to place, having coffee breaks, and knocking on doors that won't be opened. (Why did they visit me, but not my neighbor, who was obviously at home?... I always found that very odd.)
The LAST thing they want you doing, is having conversations with someone outside the organization. They need to keep you busy, too busy for your non-believing relatives and friends. Meeting attendance is essential for your spiritual growth, so they can remind you every week that YOU WILL NEVER BE GOOD ENOUGH. (Which is true... that is why Jesus died for you. You can't EARN a gift!)
Our Christian Life and Ministry Workbook for January 2016: "True Worship Requires Hard Work"
http://www.jw.org/en/publications/jw-meeting-workbook/january-2016-mwb/meeting-schedule-jan4-10/
(Old Testament)
Vidiot: I've heard that point about tax exempt status raised before. I'd love to know if that is true. I heard they used to try to collect money for the magazines. I'm not an accountant, but I did a quick search of the IRS website, looking at the requirements for claiming that you are a charitable organization. There is a 12 page form to be filed, but I can't find any obvious places to report hours or specific activities...
https://www.irs.gov/pub/irs-pdf/f990.pdf