In Bangarang John lays his hands on a level 5 warrant subject which, although it's above his level, should make the claim. This claim/contact law is discussed between Dutch and the slave fight owner. Why does Fancy Lee then try to take down the warrant? Does John not count as claiming the warrant?
If I remember the situation in Bangarang John was trying to save the guy in the warrant (his brother Davin), and a lvl 5 is "bring 'em in not dead or alive, just dead." John (and Dutch) "failed" to execute the warrant by not killing him.
That said, from what I've seen on the series it's not so much a RAC agent can't assist a higher-ranking take down a warrant, as is if they do the higher-ranking partner still gets the credit and the lion's share of the bounty (this is speculation, the show didn't given much detail on RAC procedure, but it seems to fit the bounty-hunter mentality).