He's flying the Mexican flag because his ancestors are Mexican and Mexico's team has advanced to the 2nd round of the Cup.
I mean, if America had a national soccer team and it managed to advance to the 2nd round, maybe his critics would have had a point - but who other than Mexico could a proud American root for these days at the World Cup?
Maybe one day in the future there might be an American team in the World Cup - maybe even one with a captain whose ancestors were Mexican like Senor Haro's (what a wonderful, crazy dream!) - a team that would be really easy to root for, you know, like one going into the Cup as underdogs and advancing from their group by scoring an overtime goal at the last possible minute . . . but until something like that happens, Senor Haro pretty much has to root for Mexico.
^__^