In fact, 67 percent of developers polled by Evans Data Corp reported that they spend some time developing open source software while at their primary job. This means that some portion of the salary paid to the developer is allocated to work not related to their job.
This means nothing of the sort. The vast majority of those developers are working on open source software that is used by their company with the full backing of their managers. Sure, it's a cost to the company -- open source software is not free. But to imply that this is somehow stolen from the company is just egregious.
I'm sure most ycombinator companies are doing the same thing. It's hard to be ramen profitable if you're paying $100K to Oracle for a database license. Much cheaper to have 10% of a developer tweaking rails plugins...