If a private building wishes to jam cell phones, that's their choice. No idea why doing so would be illegal as long as they made certain on the door that cell phones are jammed inside. To not do so would be unethical.
A warning should be obligatory, as there are people with health problems and the like who could suddenly depend on a cell-phone.
An emergency override button is a must, too, I think. Someone having a heart attack? Press the button and dial 911.