If the throw pulls the fielder into the path of the runner and it causes them to slow down, change course etc the umpire should rule obstruction.
I can't think of any rules that specifically mention incidental contact, but there are a few case plays that do. One is when the runner, the fielder and the ball all arrive at the same spot at same time. Assuming the runner was not impeded prior to the ball arriving it would be nothing. The other is when a batter bunts and they and the catcher make contact. In that situation the case play indicates it is nothing since the players start in such close proximity and both are doing what they are suppose to.
I can't think of any rules that specifically mention incidental contact, but there are a few case plays that do. One is when the runner, the fielder and the ball all arrive at the same spot at same time. Assuming the runner was not impeded prior to the ball arriving it would be nothing. The other is when a batter bunts and they and the catcher make contact. In that situation the case play indicates it is nothing since the players start in such close proximity and both are doing what they are suppose to.