It's not at all illogical when you consider that many with a pro account, who upload and display many GB of images and video in many many galleries yet don't make any sales, effectively cost Smugmug far more than they return to them in commission from sales, because those who don't sell, but utilise lots of storage are only worth $150/year.
I have no doubt that many pro's who do sell, and sell regularly also utilise huge amounts of bandwidth and storage space and whilst they further contribute their 15% commission, Smug can see how much these pro's are spinning through their accounts, and thereby would probably draw the conclusion that alot of them pay their $150/year + commission but if they sell at low cost well 15% of almost nothing is almost nothing..
I think it would be better for all if they just stopped offering the free accounts, and for such a great product, make everyone who wants to use it pay something to lessen the smack to the people with Pro accounts.