Please no MBAs

So I was reading Marc Andreessen's blog today and noticed a post that his company, Ning was hiring. I thought I'd poke my head in and see what they had going on and ran across a position for Product Manager; a career change I've been investigating since I started my MBA program. What astounded me was the line at the bottom of the posting:

Qualifications

• Passionate about creating great software that people use
• Experience developing and launching new online services
• Intuitive sense for feature design: ability to think through every aspect of the experience and make good decisions
• General understanding of web technologies (e.g. AJAX, CSS, Flash, Streaming Media), how they're implemented/served, and how they fit together
• Excellent communication and interpersonal skills
• Entrepreneurial spirit and pace. Thrive in a dynamic, fast-moving environment
• BA/BS degree. Please no MBAs.

I'm not sure what to make of that comment. Can anyone shed some light on why a company would so plainly reject people with an MBA? I suppose it is Silicon Valley; applying rational logic rarely seems to work in that dimension.