Answer:
Ahimsa is strictly prohibited in the Brahmana Varna because a Brahmana would be living under ideal conditions and would be using instinctive thinking, which is the ideal and the fastest kind of thinking.
Since people of other Varnas must imitate Brahmanas Ahimsa is an aim for them also.
However, lower the Varna more the tolerance of Himsa would be. Thus, Ahimsa is not strictly imposed on a Shudra.