The ideology that code should be free, not necessarily without a price tag, but the user should be able to audit the code and if needed improve upon it. And by sharing the changes, others can benefit from them aswell.
By being able to audit the code, you can make sure that the code is safe and does what it is supposed to do. It's hard to implement backdoors in the software when the code is open.
Most open source software licenses makes you the owner of the code, if the software creators are not moving the software in the path that you think is correct you are free to fork the code and make the changes that is right for you. Open source also prevents vendor lock-in where companies tries to lock you into only using their software.
Reading other peoples code can be very educational