Drag Bunt down first runner hit by throw and called out!

[rrussum]

Last night we had a man on first with my son up at the plate he drag bunts from the left side up the first base line and as he makes his way to first he is running striding with the foul line in the center of his body, the catcher throws the ball and it hits him in the back as he crosses first basse, the umpire calls him out stating he was in fair territory when the thrown ball hit him , I always thought it was the catchers responsibility to step into the diamond and throw to first at his own risk with the runner like a pedestrian having the right of way. Later I thought this to be a judgment call and after the fact I wondered should I have asked the home plate ump to ask for help from the first base ump anyone seen or heard of this rule?