ROTFLOL! And the taxman takes a lot more than 10%. In that light, it seems stingy not to give HaShem His 10%, doesn't it?
Plus, the church pays no tax. Gross does not actually reflect what you earned or "your fruits." In order to be your fruits, you would have to possess them first. The diff between gross and net is money that was never ours.
However, if they structured taxes differently, and you paid them all in and there was no withholding, then you would have to pay gross - BUT taxes would be a lot less, because people would not elect politicians taking money out of their pocket after it hit that pocket.