"They won't admit to this on their website"? I think it's more that they won't shut up about it. Their entire business model and their marketing is all based on the idea that most people are around wifi most of the time.
The cell data you pay for is your cell data usage, you (of course) don't pay them for your wifi data usage.
The cell data you pay for is your cell data usage, you (of course) don't pay them for your wifi data usage.