[rmc]

To be fair, we can also do that without source code

You can, but it's not the same.

With an open source programme:

* it's much easier to find & disable the DRM checks

* I can (legally) distribute a version with DRM disabled

* I can legally distribute a (probably small) patch that disabled the DRM, it's much easier for someone else to verify that my patch disabled the DRM, and doesn't have a trojan than if I give a binary diff.

* With open source programmes, I could submit my patched version (that disables the DRM) to a very freedom loving distro (like debian), and campaign for it's acceptance into the package index (making all Debian users not have DRM, etc.)

These things are technically possible with propriaty software, but you don't have the advantages above.