Not necessarily. While of course in many many cases, open source is a volunteer effort, there's usually some implicit transaction going on. Whether that's improving the software for yourself and passing that on to others, being a business and improving a library or something you use that helps your project generate revenue, or even a straight up commercial transaction.

But in all these cases, the open source project can be taken by you (or others) and you can do whatever you want with it. In the case of Winamp here, you cannot do any of that. It would be different if they were paying for contributions. But they're not, so.