> My workstation runs all the time and I don't care that it consumes this 100$ of electricity a month.

That has nothing to do with zero marginal cost. The term specifically refers to the cost of producing a new copy of the software, which is so low as to be effectively zero. Almost 100% of the costs of software engineering are what would be called “fixed costs” in economics.

This is to differentiate it from the production of, say, a car, where the marginal cost of building a new Ford Focus is a significant portion of the price.

> But an already well fed software engineer doesn't think that every minute of his life is precious and it costs way beyond the price of his dinner cheeseburger.

For some developers, if they were not developing open source software for free in their spare time, they would be doing something like playing games or watching movies. At least, when they use their time to create software, they can feel like they're doing something good for other people while improving their own skills - which can translate in better jobs and hence more money though I think most would do it even if that were false.