Think the immigrant thing is being exaggerated. The vast majority of guys are either born, raised, or played in the country they represent. The Dos Santos brothers for example were born in Mexico and have been playing for the Mexican National Team since they were teenagers. There is actually a decent sized Brazilian Diaspora in Mexico hence why they have a decent amount of people whose family may be Brazilian playing for the National Team. Furthermore, any naturalized citizen cannot play for a country. The rules are you can't have ever played for another country on the Senior team in anything other than a friendly.
So not sure who in particular people are complaining about as I really can't think of any naturalized citizen that didn't have a nexus to the country they ended up playing for. The most recent I can think of is perhaps Diego Costa born in Brazil who played a friendly for the Senior team but then requested to play for Spain. However, in that case, that was after he spent nearly a decade playing in Spain so it was less the case of him being a mercenary and more a case that he felt that was his home now. Further, Brazil actually had a need for a number 9 at the time so not even like he wasn't going to play for them.
If naturalized citizens can vote in a country not sure why they shouldn't be able to play on the National again.
