From Stephen Wolfram himself on his personal blog:

Why aren’t you using our technology? It happens far too often. … Sometimes the answer is yes. But too often, there’s an awkward silence, and then they’ll say, “Well, no. Could I?”

Here’s the kicker for open source developers…

If you’re making a free, open-source system, you can apply for a Free Production License.

In the license it says “Open-source projects approved by Wolfram,” which seems like they’re going to maintain a list of approved projects, but Stephan mentioned that they’re still working out the kinks in usage and licensing and they “are committed to providing predictable and straightforward licensing for the long term.”