> b.) Immigrants in prison are good, because they do work for less then minimum wage.
I think that (b) doesn't really respond to the argument. Those who worry that "the immigrants are taking our jobs" are worried precisely that immigrants will do 'our jobs' for less than we would, making it likely that an employer will hire them instead. The situation being described is precisely one where prisoners will do work for free, as a result of which no non-prisoner members of the community has a chance of being paid to do that work.
