RTE has a bit on this. The White House World cup boss, Trumps world cup chief, the US World Cup big cheese said:
"No players, no coaches have been denied," Mr XXXXXXX said. "There have been some officials that have been denied, and for good reason."
A US State Department official said last night that the referee was "associated with suspected members of terrorist organisations," therefore "making the traveler ineligible for admission to the United States".
So basically a very vague "guilt by association thing".
And who is this US world cup boss/guru that I XXXd out above.
His name is Andrew Giuliani.
Familiar name ? Any relation ? Yes, his son.
US World Cup chief defends visa ban for Somali referee (rte.ie)