I have not been able to track down the Belgian open data law¹ but it seems in principle blocking both Tor users and archive.org from access to the address book of Chamber of Representatives would not be in line with the spirit of open data. They may not have the IT competency to serve Tor users but to treat archive.org like a malicious robot is to underachieve.
¹ I can only find an old archive of the goals of the open data policy (in French), but not the law:
http://web.archive.org/web/20160416034829/http://www.digitalbelgium.be/sites/default/files/content/FR/_strategisch/_dossier.pdf
The original link was from https://openknowledge.be/ which seems to be a stale website and an inactive project. It feels like open data got started in Belgium but then the ball was dropped.