It's not arbitrary. If the primary purpose of an entity or transaction is to avoid tax, then its employment is likely to constitute illegal tax avoidance or evasion (the terms have distinct legal meanings but those are not very interesting here). Purpose is determined primarily by contemporaneous documentation.

Is there some interpretation involved? Yes, because just about any legal decision involves some interpretation and these matters tend to be exceedingly complex. By their nature, they usually involve some sort of subterfuge or concealment. But it's not arbitrary. There are laws and precedents that attempt to define exactly where the line is.

Here are some references that might help clarify further.

http://taxexecutive.org/tax-avoidance-vs-tax-evasion/ https://www.lowtax.net/blogs/The-Basics-of-Tax-Evasion-Tax-A...

I never argued it was illegal, I'm saying it's not ethical