[ericd]

No, it's because it's a common monopoly building tactic which is bad for the economy.

It's because a larger and/or diversified business can mercilessly destroy all competition in a market by selling under cost and offsetting their losses with price hikes in areas where they face no competition until the competition in the new area gets wiped out from having to sell at or below cost. Once the new area is secure, the predatory company can then charge monopoly prices in that area as well and move on to sweeping a new area. Historically this was a common business tactic and was used to create some fairly large monopolies before it was outlawed.