Think of it this way. If you would tip 15% to a waitress in a restaurant or cafe for a meal, why wouldn't you hold the same value for someone working his a$$ off for you on a hunt? All she did was deliver what someone else prepared.
On an African hunt you end up tipping the PH, camp staff, skinners, trackers, driver, etc. It could end up being several thousand dollars. On a domestic guided hunt, the guide may be all of those things rolled together, so he gets a portion for each job.
I don't go on many guided hunts here in the States, preferring DIY, but if the guy earns a tip - I make sure he gets one.