The US is one of the only countries that taxes your income even if it's earned outside the country. You do get credit for foreign taxes paid so it's usually not a big issue, but still annoying. European tax rates are very high. In fact, in a few weeks one of the major soccer leagues (Spain I think) is going on strike for a weekend to protest a new tax rate for those making over 1 mm euros, 75% is the rate I think I read.
So yeah that's definitely an issue because those players would have to be paid much higher gross salaries to compensate. The same is true to a lesser extent in the US in states that have high state income tax rates I. E. California, vs those with no state tax at all like Florida.