There is no way the UFC is doing this, don't be ridiculous. They aren't going to screw with tax fraud to save a few bucks for their employees.
The size of a company has nothing to do with how law abiding they are. If anything, the larger the corporation, the more liable it is to act immune to countries laws, bend or lobby them to suit themselves, and are generally sinkholes of corruption and collusion with individual officials via bribers er.. lobbyists.
You really think Vegas casinos are built by honest joes scrounging together clean money from their piggybanks while paying 50%-70% in cumulative taxes? I suppose the quadrillions of dollars stashed away in untraceable tax havens like Cayman Islands, Switzerland, Bahamas etc. must be grandma's $10 a months savings added up slowly over the years.
How would the IRS know if a company takes some payments in such tax havens and then uses them to pay cash for it's expenses? They have policies of not handing over account information unless a terrorism etc. related warrants are produced you know. The US has asked such swiss/tax haven banks for blanket info many times, but they've always been refused.
I know some extremely wealthy and well placed people that do this and are rich because of it. Heck, there are countries like Russia, ex-Soviet countries, India etc. where the parallel tax-avoiding cash "black" economy is estimated to be many times larger than the actual on paper GDP of these countries.
Not saying it's right, but the difference can be as much as 50%-70% of the total when you calculate corporate tax, sales tax, luxury tax, income tax etc... this is larger than most companies' operating profit margin, it's not "a few extra bucks" and it's not for the employee it's for themselves. It might very well be the sole reason the Fertittas were able to turn the UFC around and make it so profitable where so many other big players have failed.