I think you're over-egging the pudding a bit here. If people on this board are allowed to discuss evolution, then everybody is. Some people are more informed than others and I would suggest that Prof Hawking falls into the first category.
There is nothing wrong with a scientist discussing
anything, however when a scientist makes claims--particularly predictive ones--they should take care to be clear about what they are predicting and what they are basing it on. American anti-intellectualism aside, people generally do trust scientists to know what they're talking about, and when a scientist of any stripe makes a claim about the future, they are inherently using their credibility as a scientific professional to reinforce it.