Why are they illegal?  Couldn't you argue that they are only for adjusting the length of pull?
Ive seen collapsible stocks for the M-1 carbines and they are not illegal

There are combinations of features that the government has ruled in a rifle being termed an illegal assult rifle. Most common are removable magazine, collapsible stock, pistol grip, bayonete lug and flash hider. I believe that having a combination of more than two of these items will result in the firearm being illegal unless of course it is preban.

  Example: An AR15 already has a pistol grip and a removable magazine so adding a collapsible stock will make it illegal. An M-1 carbine with only a removable mag and collapsible stock could very well be OK.

  You can argue whatever you want but it won't change anything.