The Boston Globe recently wrote about people who engage in "extreme couponing". Basically, they find ways to redeem store coupons in a fashion that still abides by the rules, but they get free stuff out of the deal.
http://www.boston.com/news/local/massachusetts/articles/2011/02/27/in_world_of_extreme_couponing_goal_is_get_it_for_free/
Is these people moral, or are they parasites (because they don't actually live by trading value for value). As reported in the story, they aren't violating anyone's rights