It's not about legality, it's about hypocrisy. Trump has stated numerous times that he feels folks don't pay their fair share in taxes - from poor people to wealthy people to corporations even. And it's about his boasts of being the best businessman ever and how much money he is worth.
It's funny how people think Donald has a vision for America even though his past vision was to make as much as possible any way he could & give back as little as possible to the country that made him what he is.
The notion that any of that has changed seems foolish at best.