This is old news. The reason the state uses the word "decriminalize" instead of "legalize" is because by Federal law no state can make laws that supersede Federal laws. This can be found in the Supremacy clause of the U.S. Constitution, Article VI Clause 2. (I had to look up the Article and clause numbers, in case someone wishes to fact check me.
The reason that I knew this was because I have read several cases where people from states that allow MJ to be smoked, used or whatever, have been caught with it in states that do not allow it and PA is one (for now). The owner of the MJ that I caught on an Interstate after a traffic stop for speeding and going off the side of the highway was, "Well, it's legal in my state, so I am allowed to carry it and use it.