I think that a few people here are forgetting that unlike todays bikes, BITD suspension was an option at purchase. Konas came with P2s or Marzocchis, Orange had a choice of about 5 different forks you could have, When I bought my GT I could choose between the rigids and some Mag 21s. Many manufacturers offered their bikes in rigid or suspension guise.
The reason 'retro-bikes' don't tend to have sussy forks is because at the time, we couldn't afford them; Also, we were suspicious of the extra weight of the forks and their value in enhancing control.
Now, we can afford the forks and we can see the benefits so why not spec your retro frame with a suitably retro pair of suspension forks? As long as the forks and the frame are a similar vintage... who cares?