Most people misunderstand open source. They think it's about code, but really it's not, that is just one facet of the debate.
The core of open source is really a philosophy, and the philosophy they believe in is more along the lines of communism or to be nice socialism.
"From each according to ability, to each according to need."
In the same way communists believe individuals should not have property rights, and that all property is the right of the state. That is why they believe that no company should have "propietary code", and that all code should be open source. In other words,
the indiivudal shouldn't have their own property, and in this case the "code" should belong to everyone, the community, for the "greater good". The same reason they think IP, copyrights, or patents shouldn't exist.
They say this is really the only way to have freedom, but the truth is it's the complete opposite of freedom.
In fact, just about every logical reason they state that they are for "open source" or really a broader philsophy, they get the opposite result.
The main point here is when talking\debating to a proponent of open source, just remember the debate isn't about code.