You can still use locked parts when doing missions in custom while using the demo... also, you can save it and edit it to make custom ships, so the locked portion only limits how many modules you can use(the amount in a particular ship). i'm assuming this isn't supposed to be able to happen in the demo, using non demo units.
I didn't really think about it when I did the demo, but it probably isn't that 'bad'. The mission selection is very limited in the demo and I think anyone who really enjoys the demo will up to the full game at some point.
Besides, you really need some of those crazy exchange maps to appreciate some of the monster ships with multiple weapon systems. :)
I noticed as well. Surprised it took this long for someone to post about it..
According to Steam I still have 6 hours to wait.. Cruel and unusual punishment V.