I agree, it should be mandatory.

I was once offered a job in Germany. The company environment was english speaking so they did not make it mandatory for me to learn German. However it was mandatory for my wife to learn German language in order for her to get the residency visa. The rationale explained was that a house wife would need to interact with the local community more i.e., grocery store etc; whereas i would be working at an english speaking project so it was not mandatory.

I must say, i agree with the above reasoning.