They probably make more money by providing the service for free. Most MM trades wouldn't be worth the trade anyway. Spending 20minutes MMing even a 10m sale will only net like $.80c. It's better to expose themselves to potential members in the market and earn their individual trust, in hopes that the next time they need a service, they'll just buy it directly from the trusted MM's services.
Making some money is always better than making no money :p. If I MM 10 trades that I make 80 cents each on, it'll equal up to $8. Sure, 80 cents might not be a lot on its own, but if you think about how much the trusted users actually MM, the money adds up a lot. This is also a way to help out the newbies on OSBot by showing they're trustworthy with a free service while people like Skype, Divica, Occazn and Yoloswag have a fee in which it provides you a 100% trustworthy service.