Because we like to assume that the people we talk with are basically good people who aren't just looking for an excuse to twirl their moustaches. So you assume

Because we like to assume that the people we talk with are basically good people who aren't just looking for an excuse to twirl their moustaches. So you assume that you're arguing against "I would like to do this, but that threatens my ability to maintain my business and the jobs that depend on it. This evil act is the least-bad option."