Other than that the straight hash for a given MAC address will always be the same, whereas encryption with a key that changes every day, or genuine random new assignment every day (discard association table every day), changes every day.
However during the trial when they used a hash, they actually salted it (as reported anyway). So if they changed the salt every day then even hashing would give comparable results.
I wonder which MAC address is being hashed anyway. The MAC address of the sending device (which due to MAC randomization might not be useful for their purposes) or the MAC address (BSSID) that it is probing for? In the latter case there may be many such BSSIDs per sending device.
This is the fundamental problem with government - there is no trust and there is no verify.
If I went to the UK (fat chance of that right now) and I had my L5, I would definitely use the WiFi HKS.