They had other players, like Norlinder, who were younger and potentially not prepared for the smaller ice and more physical play miss time and lose their place on their European team. Once a player is off the power play it can be hard to get back.
What is the benefit to bringing Reinbacher over this year? Meeting him, I guess, letting him see what NA hockey is like. Making fans happy.
He's still pretty young so bringing him here to play against bigger, stronger guys who play a more physical game is a risk. There is also the chance that someone takes his spot while he is away and that he doesn't reclaim it. As he will clearly be playing in Montreal before too long Kloten has to be looking for a replacement and there is no reason for them to prioritize his ice time if he's not there for the preseason.
I don't think it's necessarily a terrible move to bring him over, who knows, he might force them to keep him in the NHL, but it hasn't worked out very well for their other Euro prospects so far. If they are going to disrupt his preparation for the season with his team, the few weeks they have to plan strategy and set up lines, they should be getting something big out of it. Showcasing their first round pick is very much a Bergevin ego move rather than something logical and thought out.