See no one has mentioned the questionable practice of the buying and selling to yourself that every exchange I have ever tested it on allows you to do. A nice bonus feature for the market manipulators on all the exchanges.
Anyone could easily get around a rule like that by using multiple accounts. It'd add complexity to the system and only effect honest users.
I've never tried to manipulate things that way (I don't think it would work too well), but since everyone knows it possible and can do it themselves I don't see it as a problem.