Let's say person X has lots of bitcoins (potentially mined from the early days, when it was very easy to obtain). They go and sell it for $30,000, in cash. Let's assume X has the intention to declare the gains in their upcoming tax returns.
Why this would be money laundering?
Basically X is selling a property in the form of a mathematical equation, that was obtained legally. Of course they'll have to pay taxes like in any other profit-generating transaction, but other than that I'm failing in seeing the issue.
ps: it seems that in this case they were trying to entice the guy with stolen credit cards. As that didn't work, they tried switching to money laundering.