I believe that the issue with allowing negative reviews is that it would open up Dave and Trish's company (Dash Works) to possible lawsuits filed by the discredited companies. Smaller companies, like Dash Works, usually can't afford a big legal team for protection or the costs of going to court.
If memory serves (I could be wrong,) it was that exact set of circumstances, involving negative reviews on the Garden Watchdog, that forced Dave and Trish to sell Dave's Garden to a big company that had a big legal team for protection. I imagine that they aren't eager to repeat that experience.
