His bounty is only 500 bellies, which is pretty small… for anyone really.
Considering that the Marines did not recognize him as a Mink back at Sabaody and pointed out the "mystery" of him being a talking bear, I kinda assumed they just view him as that, a bear that happens to talk, and thus, an exotic pet.
They don't even notice that said pet beating the shit ou of them. These marines man… I can understand Chopper, since he transforms and rarely uses Brain Point while fighting, but Bepo having such a small bounty is just weird