A lot of people would consider that flatly immoral, and as evidence that the poor aren't paying their fair share.

Remember that Flat Tax is also called Fair Tax. The concept of fairness is central.

If everyone receives the same tax free allowance then it certainly is fair. Fairer and simpler.

I think what msla is saying is that people who are at or below the threshold for taxation would contribute nothing to taxes, which would be unfair in theory because they consume public goods and services like the rest of us.