There are many different open source licenses, and what counts as "open source" can depend on your definition. In theory, you could write a license that makes the source code freely available but restricts binary distribution, or excludes certain groups or use cases from using the software (like not allowing AWS to resell it as a service). Some would argue that's no longer truly open source, but legally, you can add almost any condition to your license.

In this case, it sounds like they're charging a fee for their pre-compiled binaries and possibly using an end user agreement to restrict redistribution. But since the source is available, anyone could compile it themselves and share the binary, unless the license specifically forbids that.

Realistically, though, many people who want the software will just pay for the convenience of the official binary rather than go through the hassle of compiling it or finding someone else who did. So, while the situation is a bit unusual, it doesn't seem like a major issue in practice.