> Is the illegality of the other activities really relevant if they can't be prosecuted?

Normally, no (which I said explicitly in another comment here). In this case, the activities are linked. There's no need to evade taxes if your income isn't illegal.

Even so, I don't subscribe to the argument that it's entirely a good thing that we prosecuted Capone for tax evasion. We were unable to prove the actual criminal activities, so we instead looked at the fruits of those activities and used that.

Another way to look at it is that the justice system completely failed initially. We couldn't build a case for the major crimes, but we could for the lesser crimes. He was still guilty of both (this is of course assuming his guilt).

In Shkreli's case, we failed to prosecute originally because it's not a crime. The current case isn't related in the same way, and linking them and punishing in the latter because of people's perception of the former is akin to levying a harsher sentence on someone because of their interracial marriage in a region and time when that was legal but frowned upon. Social pressure unrelated to the issue at hand should not be relevant in court. Even if it's related to the issue at hand in the case, it should be restricted to sentencing (where it likely can't be removed anyway).