I don't think PBMining is under any obligation of paying with freshly mined coins (Does HashNest do that for example?). As written on the site itself, PBMining consolidates from various sources, including hardware running on different pools, as could be found earlier, as well as rented hash capacity.
Pretending that, because payments are not freshly mined coin there is a fraud, is therefore abusive.
You can trace incoming payments from old customers to payouts to new customers. That's what a ponzi is. It's that simple.