Sometimes you can "buy" the coins with BTC by renting a rig. If the cost of "buying" by renting is smaller than market purchases, then it is cheaper to rent.

Please remember that unlike dedicated miners, the renters don't have to invest into the hardware, take care of it, tune it, RMA it, pay electricity bills, deal with the heat exhausted from them, manage pools and exchanges, keep accounts and do calculations. They just click a button, pay something and keep an eye on the pools.

Miners also "buy" bitcoins with electricity. They rent their rigs and they get say 1 BTC a month, but their electricity bill is 0.5 BTC that month. They effectively pay 250$ for each BTC. Would it be cheaper to buy coins with dollars? No. Or maybe yes, depending on how much BTC they get and how much power they use.