"We never sell your data" is a meaningless statement. It's the online service equivalent of saying that a bottle of water is "fat free". It is consistently and prominently written because it is a definitive statement. Facebook used to feature it on the signup screen with some other statements that sound principled like "We don't charge for Facebook, and we never will", etc.

But when you parse out what it means you are left with something more accurately stated as: "<Company> will not sell its proprietary assets."

It sort of like the operator of a hotel saying "I don't sell rooms".