The difference is equal and fair opportunities.
Super cup is entirely player on neutral stadium. The league is home and away.
If you take a game in USA, then you disturb that balance. Essentially Villarreal gets one less home game, while Barca gets one less away game.
This is just semantics, friend. The fact is, is that Barca and Madrid will both effectively have home games in the semi finals and Madrid will have 75 percent of the stadium in a Classico final.
Bilbao have given up having 25 thousand of their fans in a true neutral venue like Seville, for having 500 fans in Saudi. And they do it willingly, all for the almighty petrol dollar. Just like every other La Liga club, that hypocritically opposed the Miami game, would if they were given the same opportunity.
But apparently Villarreal can't do the same in Miami??
All of this under the strange backdrop of Barca fans opposing the game out of loyalty to a League that shows them none, and actively works against them.
All a hypocritical, bizarre farce of a situation, in my honesty opinion.