Handling physical goods is an easy way to get caught. Bitcoins are a very effective way for criminals to cover their tracks.
They would still have to buy the BTC using the credit card, although I agree it is not as easy to trace as physical goods, the eventual BTC for physical transaction could be traced all the way back to the credit card payment for BTC.
So could be a case of security by obscurity here?
They wouldn't be buying the bitcoins, they would be mining the bitcoins -- and newly mined bitcoins don't have any history showing who paid for the GPUs which found them.
I don't see how paying for the GPU instances using a credit card is any more anonymous and less traceable than paying for physical goods?
Sure mine the BTC and it won't be possible to trace back to the machine.