Penn has clarified what he means by "dangerous." I don't have a source for this, so take my memory and my word for what it is worth, but their definition of "dangerous" is something that could result in an overnight hospital visit. Cutting yourself is ok, even a stunt gone wrong that breaks a (non-serious) bone. But anything potentially serious they would consider to be unethical.
The other reason I don't think that the glass is "fake" is that Penn began his career in entertainment as a juggler. He also eats fire. Juggling knives, fire torches, glass etc. comes with some risk but not serious risk. The worst of all of the above is potentially fire eating and that's only if you accidentally inhale and burn your lungs with hot vapour, which can be serious but avoiding that is everyone's very first lesson. Juggling glass could result in a cut, even one that could potentially require stitches. But it's not a serious injury.
Several folks also mentioned that his definition of being too dangerous would be having to stay in the hospital.