Does your real estate agent take a percentage of your profit?
Does your broker base his fees on your profit?
Does your property tax bill ask what your profit was?
Of course not, so why is the US Government that? The value you get from the government running the military, maintaining safety programs,and building and maintaining our transportation infrastructure isn't based on how much you made last year, it's a fixed cost. Change to a gross receipts tax and every dollar you receive is taxed at a fixed rate. No worries about what is deductable, or what does or doesn't qualify as pre or post tax. Plus it's more easily auditable.
My town uses it and it's pretty fucking straight forward. You put down what you grossed, and you multiply that by between 0.10% and 0.37%. Yes, you read that correctly - our local business tax is ten to thirty-four CENTS for every One Hundred DOLLARS you gross. And, aside from lying on your tax forms, there's no way around it.