Even with monopolies... true monopolies don't exist under a truly unregulated market - you get companies like Coca Cola or McDonalds who dominate certian sectors, but they never have complete control of a particular industry.
I have to take exception to this. There is a tendency, particularly at a certain stage of growth, for some enterprises to consider methods of unfair competition.
Just look at the drug trade. Rather than reduce prices to get more customers, the pusher considers other techniques to reduce competition.
Unfortunately, an even larger entity, (the state), often has to provide a regulated environment, or the pushers eventually will become the state. (See South America)