Well, stirring them up while you work there would be grounds for being fired. When do you think people can talk about it? If you shouldn't speak ill of your employer while employed and you also shouldn't speak ill of your employer after you're no longer employed by them, does that mean being hired means you should never speak ill of that company again?

They made it very clear that while they've held these opinions for a while, they've avoided situations where they would be presented with either lying about their opinions or bad-mouthing the company they work for whether out of respect or fear. I think that's as professional as you can expect people to be. If, as a company, you expect people to not speak about their subjective opinions after they are no longer getting paid as an employee, you need to be willing to offer them something for that, because what you're really asking for is an NDA.