I'm replying without reading through the whole thread; however, a similiar letter came from another NC district a few years ago. HSLDA advises against overcomplying with the law because it essentially gives legislators and PS administrators an inroad for demanding additional information from all.
From my understanding, it's sorta like a fence line. If your fence is on your neighbors property line, and it goes uncontested for several years, the neighbor is viewed as having deeded the land to you - defacto. When enough people give information that isn't mandated, the weight shifts and now is seen as "missing information" from those who were never required to give it anyway. A case can be made for policy/legal change.
Giving additional personal information, reporting test scores, or reasons why you homeschool adds leverage to the side that will always want more, more, and more information. Then what happens when the information is obtained? It must be evaluated, and in order to evaluate a standard must be applied. Who makes the standard? Presto - you have lost some rights.
I believe the motivation of your district is exactly as stated/implied. They want to know why parents don't choose the public system. However, complying with their request has ramifications unrelated to their motives.
Toss it and advise all the homeschoolers in your area to do the same.