The US probably has a tax treaty with Brazil so that he gets credit for that tax paid on his US return at the end of the year. If there is no tax treaty (which i doubt) then they are really getting shafted.
Also, the reason why fighters may get behind on their taxes if due to lack of financial planning. The fighters are being treated as contractors instead of employees so there is no obligation from the UFC to withhold any taxes and remit them on the fighters behalf. The fighters might get a hundred grand cheque and think it is great and go buy a 100k sports car. Then at the end of the year when they do their taxes the find out they owe 30k they don't have.