Kickstarter does it every time they suspend a campaign for being impossible, (Look at skarp for the biggest example, they gave up hundreds of thousands of dollars) which happens fairly often. A couple times it has been a result of the campaign being posted here.

Please, don't insult me. What Kickstarter does is hold backer's money until the campaign is finished. Upon completion, the money is transfered to the project creator. If the campaign is suspended BEFORE completion then off course the backers will get their money back since no money has been transfered to the project creator.

Now take a campaign that has not been suspended and is successfully funded. If the project creator just takes the money and run, Kickstarter will not do anything or refund the fee they took on each "donation".

So no, Kickstarter does not do this everytime. Kickstarter makes money on successful campaigns, whether it turns out to be a scam or not.

Indiegogo's rules are basically limited to not claiming to have created someone else's product, and beyond that almost anything goes regardless of it being physically impossible

Is even worse because it has "flexible funding".