Q: I notice that the pitchbend wheel on my keyboard springs back to center when released, but the modulation wheel does not.  Is this normal?

A: Yes, this is normal.  The pitchbend wheel is used to simulate bends, such as bending a guitar string, which would return to a centered position when relesaed.  Modulation is a parameter that can be adjusted and set at a desired level and is not supposed to automatically spring back.