The purported reason is because the survey was designed by a private company who claims the survey to be their intellectual property. Similarly, if a school screened a controversial film, a parent could not ask the school for a copy of said film, because the school doesn't own the intellectual rights to it.

This is a microcosm of what happens when public institutions, whose records and dealings are considered to be part of the public record, uses private contractors. Other public instituions have used this to deny access to what is usually a citizen's right to know: e.g. public worker salaries being maintained by a private payroll company.