Yes, a hospital is just like any other business / property owner.
If they post, they lose immunity from liability. See 175.60(21)
Normal people look at that & say, well, if they lose their immunity from liability then they're liable, and since they put their customers in a dangerous situation, it's their fault when the customers get hurt.
But oftentimes lawyers (including lawmakers & judges) don't think like the rest of us.
There hasn't yet been a court case to confirm that places which post (Azana salon, Potawotomi casino, churches, hospitals) are actually liable for damages for not protecting people they required to be defenseless.