The commercial button mushrooms are still carcinogenic when eaten raw. The reason you see them at salad bars is because in practice there is no way to sue the salad bar if you get cancer 20 years later or whatever. But it's still not a good idea to eat them raw.

The only two mushrooms people generally recommend trying raw are the beefsteak polypore and the caesar's amanita. But this isn't because they are healthier when eaten raw, but rather because they have some unique culinary properties that are considered worth the risk when consumed in small quantities.