The reason you do money laundering is because the source of the funds is illegal. If the source of the funds is legal, just claim it. There are plenty of occupations that get paid in cash and are expected to report it.
The IRS isn't referring suspicious (whatever that means) tax returns to the authorities. What happens if you are a criminal is that the authorities have there attention on you because you are doing illegal things. One angle of attack for them is your finances. That is why money laundering exists.
Maybe the reason is the other way around. To convincingly wash money you need a legitimate looking shell business. And it needs to pay tax for the reason any other bsmusiness does.