To say that a bounty campaign extended is a scam may not be totally true,because there are many reason why a team can decide to extend the period of the bounty campaign of their project,one of which is to buy them time to be able to reach their target.If such things happen,we just have to make do with it as investors to be able to have a successful campaign.
I haven't seen a snatch that has been extended and scammed people. I see the team as the reason for extending the project, we can not do the job anyway, then we can collect as much money as he says.