When you release a licensed pinball there are certain requirements with the license holder, in this case Lucas (or Disney now). One of the requirements is that they get to review the final product and have final approval.
They don't wan't people modifying or distorting their brand, they are trying to protect their IP. It's very possible that Williams or Stern don't give a shit as you put it, but the license holder of the theme does.
Letting people write their own code will probably violate the license agreement. Look at WPC 2.0 which is exactly what you want. It's not planned to work in any licensed games.
Quoted from CaptainNeo:What I don't get, is why would anyone even give a shit about what software you run in your game. If anything, they should want software upgrades because it gets people to actually desire games more. more desire means they will spend the money to restore and keep it. Which means, more parts to buy from them. There never has been any money in the software. They dont' make dick on it anyway. So why would they give a shit? personally, I think they are shooting themselves in the foot by restricting custom software sharing. I think IJ sucks balls. But with a software reboot, it might be awesome enough for me to actually search for one. But if I can't get the code, it will still be on my low 50's list.