Policies on unsolicited material are not only common-place but just plain smart on any company's behalf which releases any material that one may consider intellectual property. The reason being is protection.

If some crazy loon walked in and said "I think it would be a great idea if Hildebrand had the power to turn into a unicorn!" I'm sure SE would usher them along their way and say "yes yes that's nice, we'll think about it"

Then, if a year later, Hildebrand suddenly had the power to turn into a unicorn and our crazy loon had the ability to prove he provided this idea to SE, he could sue them... unless of course they had a legally binding policy in place regarding unsolicited material in place.

My point: I'm not surprised they have such a policy.