Taking bribes was unscrupulous, but let's not overlook the fact that, with or without bribes, non-Roman heads of state (i.e., client kings) typically had patrons in Rome.
Plus in Jugurtha's case he was connected to the Scipionic family having served under Scipio in the spanish Numantine war. With such high connections and natural cunning bribing wouldn't have been hard. Jugurtha is also to have said of Rome "Oh city you would sell ourelf if only you could find a buyer" Infact bribery seems to have been a peice of cake!