First, because it reads like a tacit threat - even if you aren't personally a convicted felon, anyone reading this might find themselves less willing to even consider someone who is.
Second, people adjust their expected salary expenditures on any number of qualities and characteristics. It's not "taking advantage" of someone any more than paying a junior developer less than a senior developer. There's increased risk - real and/or perceived - involved in hiring someone who has had difficulty staying within the confines of the law.