Erm, thanks but no thanks

Being invited to attend a company's "Customer Advisory Board" is nice.  It recognises my real-world experience with their products, and hopefully means some of my ideas will end up in the product, making my work life easier in the future.  I'm not surprised that I've been asked to sign an agreement, since I'd be giving them ideas that might end up in their product. However I most certainly don't expect that agreement to include the following clause:

You further agree that you shall not make, via private blogs or any other communication method, negative or misleading comments about <company> or our products

If that's the way you want it, you'll have to live without my attendance.