The reasoning for this is slightly complicated but it’s important. When players are drafted in the NHL, the team who drafts them owns their rights, meaning no other NHL team can sign them. The length of time that a team owns a players rights is determined by where the player played prior to being drafted, it’s an odd system but it goes as follows.
If a player played college hockey in the US, (Troy Terry, Jimmy Vesey, Justin Schultz) the team who drafts them will retain their rights until 30 days after they leave college.