I think you maybe confusing anaerobic capacity and local muscular endurance.
Your right in the regard that the legs get better at sprinting (ie muscular development and lactate flushing during the rest), but lactate builds in the body as a whole, so by sprinting your developing the body as a whole to cope with the stresses. True when punching you need to develop the local endurance in the arms, but thats through pad work and/or punch sprinting. The fact is because the legs are so big incomparison to the arms, the lactate they produce in sprint work is much greater then that of the arms under the same conditions, so the adaption to the lactate is much more wide spread when legs are used rather then arms.
I could be wrong on this, but I don't 'think' I am
Also, glad to see your asking good questions now toufeksian, you asked some shit ones previously