There wouldn't be a second transaction (the front store) if it weren't for trying to pay taxes on the first. So that's not really a defensible line of reasoning. Only, perhaps, if the tax rates would differ.
Sure, so if you want to use circular logic to say the real crime of money laundering is tax evasion, when money laundering actually results in more taxes being paid, that's fine. But don't expect that to be a persuasive moral argument for making money laundering a crime.
I didn't say the real crime of money laundering is tax evasion. The real crime of money laundering is obstruction of justice. But taxes are certainly being evaded, too.
You are paying some tax, just not all the required tax. Specifically you are evading income taxes. As I recall in the US at least, there is an option on your IRS form to report any income derived from illegality so that the relevant tax can be paid. Whether anyone has ever done so I don't know but something like that could have kept Capone out of prison.
If you illegally sell $10 worth of drugs, then record your front store of selling $10 of Tylenol (that you don't actually buy or sell) then the $10 of drug money has been fully taxed.
There's taxes from the supply chain of Tylenol that don't get paid when it's $10 in illegal drugs that are actually sold. Much like Bitcoin, with a black market you can only tax at the egress points when the money needs to touch a regulated financial entity whereas an all-legal system can tax any time money changes hands.