I think they’ve already done that. It wasn’t oversight or misinterpretation, it was deliberate fraud.
Yes, and no. People can talk themselves into an awful lot, and tell themselves that they have a clever lawyer who found a perfectly legal loophole they can use to hide their income from taxation. That's why the IRS conducts audits. They know that people are claiming deductions and business expenses they aren't really entitled to. In the case of me and my 20+ colleagues, I would talk to them and when I counselled them to set money aside to pay taxes if the IRS caught up to them, they would, in all sincerity, insist that the payments made to us were all perfectly legal and not at all taxable. Before I went to a lawyer, I went to a tax preparer, and she insisted that none of it was taxable, even though I explained exactly my real situation.
It will be for the prosecutor to explain why this was more than just that sort of wishful thinking, and for a jury, and then an appeals court, to agree.
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Also, why would they have to prove that? Is other people have committed similar crimes and not been prosecuted a valid defense?
It can be. Selective prosecution is grounds for throwing out charges or overturning convictions. If a judge can be convinced that this is not normally prosecuted, and that this prosecution is simply politically motivated, they are very likely to throw it out.
Again, for emphasis, I'm not saying that this is an example of selective prosecution, or that the tax cheating involved here is not the sort that shouldn't be prosecuted. I'm saying that the defense is going to try to make that argument, and if they convince the judge and/or jury that it's true, then they won't be convicted, or their conviction will be thrown out on appeal.
This subtopic started when I responded to a question about prosecution for tax fraud, and I was noting that a discovery of tax fraud does not always result in prosecution. Sometimes it does. Sometimes it doesn't. In this case, the prosecution thought it should. The defense will try to argue that it should not.