Because for one reason or another trapping is a fact of life, and it's safer for domestic animals when it's done responsibly.
For example, a body-gripping trap that's set in accordance with current Minnesota regulations (including new, non-functional 2012 regulations) at ground level humanely takes its intended targets including raccoons, fishers, martens, bobcats, etc.; but it also tragically kills domestic dogs--very quickly, despite their owners' efforts to release them. When a responsible trapper takes measures to protect domestic dogs, that same trap in different sets (or different types of traps in different sets) takes the same intended targets, but spares domestic dogs. Those measures take extra effort, so they need to be supported.