Pushing a round from a mag adds substantial resistance to the chambering cycle, drastically slowing the velocity of the bolt. Think about it, the bolt has to overcome static force and accelerate the shell and overcome force of friction holding the shell against the magazine lips, it then has to push them past the lip and angle the shell into the chamber, where the shell is forced to straighten out and is slides across the face of the bolt into the chambering position where it is finally driven forward and the bolt locks. However, if the bolt is flying forward only the minute force of the extractor snapping over the rim and the regular friction of the sliding bolt assembly oppose it's forward movement. The firing pin could potentially gain enough momentum to slam forward against the primer and set off the shell, and it may do so when the gun is unlocked, depending on the design.
Most bolt-action fire arms will "shoot themselves closed" wherein closing the bolt partway and pulling the trigger sends the firing pin forward while simultaneously locking the bolt, and assuring the gun is mostly locked up before the firing pin can even contact the primer. Another example is the AR15 where the firing pin cannot touch the primer until the bolt is 100% locked. Of course this doesn't necessarily prevent a slamfire, but it does prevent an out-of-battery detonation.