In the long run it is bad for consumers. If you sell widgets on amazon for $10, and amazon charges $2 or whatever, you pocket $8. Maybe you also want to sell your widgets on your own site where your overhead is only $1. The "most favored customer" clause prevents you from passing that savings on to the customer and charging $9 on your site (or any marketplace where you might prefer to sell things compared to amazon). It promotes stasis or growth of large sellers and prevents new ones from offering price competition.