Some teams have decided that it's better for Euros who really aren't ready to just stay in Europe and do training camp with their team. Having them go to NHL camp, then go back to a team that prepared without them can set them back. For example, the Canadiens have explicitly said they think they've made that mistake with a couple of prospects and feel the continuity and comfort during a season of development is worth more than "getting used to North American hockey".... especially when training camp isn't really North American hockey.
I'm not saying those teams have the right philosophy, only that the "Hey let him come to America for a month, if he doesn't make it, nothing lost" isn't universally held by some of the people who do this for a living... right or wrong.