The only reason I can think of for why it isn't public, has to do with so many athletes having their hat with so many sponsors. A lot of times, these are paid sponsorships other times simply just free stuff from that company. I suppose when an athlete signs on with a new sponsor you could include a blurb on what they are getting.
But then the other problem I see is athletes using that as leverage, because running is solely individual performance based. Imagine athlete A saying I beat Athlete B in 6 races this year, but Athlete A ends up making the OLY team.
Anyway, I wouldn't be surprised if some of the more veteran athletes like Symmonds, Goucher, and Flanagan make over 6 figures.