I tip on hunts because it has become part of the expectations.
Tipping in general has gotten out of control. Servers in restaraunts is different, they essentially don't get paid, they make about $3/hour, well below minimum wage and close to 100% of take home is tips. Essentially they make enough on "hourly" to cover taxes on the tips.
On a hunt you are paying somewhere in the area of $1000/day or more and that SHOULD be enough to fund everything. But tipping is part of the expense you should plan on as it's part of the expectations now. Guides do not make much money.
