Do they understand for example that the depreciation deduction for property rental has no bearing on available income?
On schedule C they should no more deducting mileage on a private vehicle than a W-2 worker should be deducting mileage for driving to the office.
Successful self employed people, real estate investors, farmers, investors, etc, have little taxable income.
What should matter to DoS is cash flow to pay expenses. For W-2 workers, gross income is pretty much cash flow. For everyone else, DoS uses taxable income, which is meaningless in real life.