As I understand it (and obviously open to correction), SRS felt that people were getting "monetary reward" by posting numbers with rifles they were selling and SRS wasn't getting their cut. I agree with Dick - the numbers were a taste of honey that probably generated interest in getting letters.
I always tell people that SRS is a "snapshot". While a certain M1903 (or Krag or M1 or M1911) might have been assigned to the 7th Cavalry at a certain point in time, a year later it might have been assigned to a soldier guarding a warehouse in South Dakota.
"We make men without chests and expect from them virtue and enterprise. We laugh at honor and are shocked to find traitors in our midst."
--C.S. Lewis