There's a feedback effect where those developers get hired and then become the hiring managers (employers). So the line between developer and employer is gray enough for some blame to lie on both sides.
So, I'm old enough to remember a time before widespread open source and even before the world-wide-web itself. I was paid to work on whatever my employer thought they could sell, but not necessarily what I was interested in learning more about. When Linux came out, for example, I thought that going through it and maybe even working on it would be a great way to learn more about operating systems in a way that I would never get a chance to do professionally. Being curious about something isn't necessarily declaring it "worthless".
That said, I agree with the author that it's scummy to demand it of employees (although I've never seen that myself).