Microsoft unfortunately doesn't allow on-device mining in Microsoft Store apps, so the ads actually are paying for cloud miners which GroupFabric runs. I.e. the mining isn't performed on your machine - GroupFabric is running the miners already on your behalf.

The ads are what pay our power and infrastructure costs which is why the loss of ads is bad for yields. We're working on moving Bitcoin Miner off of the Microsoft Store so users will be able to mine using their own machines.

Once users are mining on their own machines, we'll be able to offer a no-ads option in exchange for a fee % of what is mined as mentioned here.