The Prosecutor had a search warrant. That had to come from a Judge. I fail to see what upsets you about investigating a business that was engaged in covering up illegal activity?
First, they were not investigating the business and really could not. Not for preparing tax returns because preparing tax returns is a lawful business and everyone has to submit tax returns and everyone therefore has the right to counsel and advice in preparing those returns. Everyone whether the income is legal or illegal is required to pay his taxes on it. The process of payment of taxes cannot be used as evidence of another crime. This is a basic common law principle and an exceedingly important one.