If they think Bruce can bounce back offensively, and don't have to add payroll or trade prospects in this swap, then what's the big deal? Saunders is a year removed from knee surgery and missing all of 2015. There's no guarantee he's going to be healthy or good. If they think Bruce's offense can be fixed, and that's all they have to give up to see if it can, then it's not worthy of negative backlash.
If no money is involved, or the Jays are sending meaningful prospects, then that's a different story.