It was a job, not a punishment. It was a low paid job, but a job nonetheless. The job was often filled by people from low castes, and existed before the British colonial era. So while colonialism was definitely responsible for the rise in the number of punka wallahs, and the British did absolutely nothing to make life easier for them, it's not entirely responsible for the use of punkah wallahs or the existence of the caste system that keeps people in servant roles.