I was looking at that too... the trick that I see is that that's a statistical limit, not a "travel X miles, lose X% energy, no matter what" limit. It's still entirely possible for a particle to be over the limit, it's just highly unlikely.
We detect super-energetic particles frequently, but only a couple "over the limit". Seems to fit the statistical model to me.
Anyway, if you propose that this is the mechanism for energy loss and you know how dense the CMB photons are (which we do very well) then you can predict interesting things like the maximum distance a cosmic ray can travel before it runs out of steam entirely, see
http://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit
Although i'm not sure how the OMG particle fits in with this scheme yet