It isn't thought-policing. Everyone is free to have their opinion. They are even free to share their opinion. But, if you broadcast that opinion there can be consequences to doing so.....up to and including pissing off your employer to the point that they eliminate you.
As an employer I don't need my employees speaking to the media (unless that is their job function), offering negative public commentary on how we handle things within our business, or generally sharing any information that they have not been authorized to share.
When you do so orally at a cocktail party, the chances of it getting back to the boss or causing damage to the company (real or perceived) are pretty minimal, so your risk of consequences are minimal. When you decide to tweet it to the world, post it on your facebook page or feed it to a reporter your risk has increased exponentially and shows a pretty striking lack of good judgment. Unless you bring something to the table that is exceptionally unique or can't be found in a replacement, why should an employer tolerate that?