For me, it is their right to do so. Whenever we join a bounty campaign we are bound with the rules in it.
Sounds like bounty hunters have to do a better job at vetting their ICO choices.
So for you people it's ok to say that a bounty campaign will run for 4 weeks and you'll be paid 100 tokens (just as example), then before it finishes they tell you they're gonna extend it for other 6-8 weeks without any extra reward and if you change the signature you'll be disqualified and then, few days before they give you the reward they tell you they decided to give 50% of what they had previously promised? So you worked the triple of the time for half of the reward? Wow, probably you people work for the glory, not for the money, I really admire you and I really hope you can buy a lot of food with that glory!
I think we should try to see this from a different point of view: do they actually decide these kind of stuff at the end of the bounty (make it longer, reduce the rewards etc) or maybe, I say maybe eh, they decide all this before the bounty begins, they write their "we can do whatever we want" clause and here we go! They cheat a lot of people make them believe there is a short and/or rich campaign so they have a lot of people joining them and then they change everything, and the poor idiots can't do a shit. Am I really the only one who thinks this?