It's a kind of interesting point. At the time ADS-B was conceived they would have been aware of the lack of security I'm certain. However, the privacy element has, for the most part, only become relevant because of practically disposable Software Defined Radios being mass manufactured (actually repurposed TV dongles). At the time they created the spec I'm sure they considered the cost of buying a niche set of radio hardware a barrier to the general public having access.
