One of the core ideas of Communism - workers own the means of production. Source code is means of production for software. Capitalism is when the source code is owned by the "factory owner", i.e. corporation, and not by the people who actually produce it.

Example: programmers who work at Microsoft don't own the source code and can't use it for their own benefit, while Microsoft makes money from it and only pays wages, which are below the value of source code to the company. This is the old model of Capitalism.

Open Source is very much a Communist idea. People who work on open source, even if they are payed for it, still have access to their work and can use it independently of the company.