This person is suspected of selling Bitcoin for cash.
It says so right in the article:
"undercover cops sought to identify the individual behind the online moniker “elonmuskwhm,” who they suspect of selling bitcoin for cash"
Says it right there.
To anyone who thinks that is acceptable or that there is some sort of overreaction by the authorities, I don't even need three words to refute you. Three numbers is enough:
9-11
There must be more to it than that; buying and selling Bitcoin isn't illegal.
Although the thread root refutation makes no sense. Freely trading bitcoin leading to 9-11 is going to have to involve some flexible mental gymnastics since 9-11 happened in a pre-Bitcoin world. In practice AML style laws are generally targeting tax evasion.