In a free market system people can transact as they wish, including giving away something for free if they want.

There is nothing at odds at all. If you don’t see it, you might have a rather cartoonish, villainy view of a capitalism that gets promulgated by people who refuse to allow anything good or nice to be ascribed to capitalism.

If you can’t understand why capitalists can also like open source, have you considered that maybe it’s your understanding of the system that is flawed, not theirs?