If they buy there is no payments to send out, if someone else buys they send payments to someone.
But isn't this part of the major gripe here, is that the payouts are low after the fees? With a virtual miner they can literally pay you anything (within reason) they see fit. Oh... BTC/USD had a bad day... your multihashlet is now in the red... here's a satoshi to keep you warm and fuzzy... always profitable!
They don't have to have an inventory to keep on hand, so buying back doesn't do a thing for them.
I get your argument... but it doesn't make sense in the virtual miner world.
I think it is primarily on ZenHashlets and Primes that they do it on since they are really the only ones I have heard this happening to. Primes since they can mine anything, boost, and double dip. Then Zens since they mine a higher payout pool. By GAW buying all these back at such a low price they don't have to payout BTC to those sellers anymore.