Spending thousands of dollars to avoid their moral tax obligations instead of paying is in my view fraud and corruption; the fact that it is possible with an army of accountants and massive resources dedicated to tiny loopholes is really of no importance when we weight the ethical or moral judgements of these actions: if you spend millions on lawyers to devise a clever loophole that lets you murder someone legally you are still a murderer. There's a (bad) argument to be made that for consistency's sake we cannot prosecute you under the current laws, but there's no argument that you're not a monster.
Refusing to pay taxes is stealing from the poor (and the general public) who rely on government services; you are destroying the roads, parks, annihilating schools and universities, preventing people from getting healthcare they need to survive, stealing from children, the homeless, the elderly and the disabled, helping criminals by defunding the police and helping burn people and property by defunding fire departments.
If your job is to find tax loopholes for corporations, you are committing fraud even if our current legal system is written by people who think fraud is okay so long as it's a CPA on behalf of a massive corporation doing it. The fact that some judges have decided this is okay does not mean that it actually is.