Why you need the source code
I am only a civilian software engineer - don't want to intrude too much but I can say why we need the source code.
There are always specific needs for each user that have to be customised. This often involves writing programs that interact with the system that you have purchased.
Usually one expects documentation to tell you how to do that but documentation is as hard to create as the actual software so there is never ever enough of it in any rapidly developing advanced system. In cases where people are not sure how to do something it's great to have the source code as a reference - in fact essential. It tells you what's really happening instead of the bull**** that the docs spout.
Code is always full of bugs - especially code from big companies. So being able to discover why something isn't working yourself without waiting for some overworked LM employee to get back to you is important.
It is scary for any company to hand over this kind of stuff but they want the defence contract and they are getting paid very well indeed so I think they can **** hand it over.