First base is completely in fair territory. A straight line from the right handed batter's box to first base is in fair territory. The only part of the base that is included in the running lane is the width of the foul line. The running lane is illogical with the field design and speed of the game. Therefore, the interference rule upholds the spirit of the game, but has been poorly implemented.
However, It was a good call based on the rule and the occurrence. Sad to see in a WS game, but it happened.
BTW . . . Gurriel covered first base like an uninterested tee ball player. No right foot on bag? No stretch? How bad do you want to get someone out?