It simply amounts to a basic understanding of the relationship of supply/demand/price. Businesses coming together to set prices distorts the constraints of supply and demand.
Edit: And not just supply and demand, but also productivity/efficiency. A more efficient business may still have the same constraints of supply and demand as their competitors, but may be able to offer a lower price due to productivity gains. But, if they collude with their less efficient competitors and raise prices, then again, another market distortion.