Renaud Lavoie: The Players Association filed a grievance when the NHL decided in 2015 to impose a compensation system (choice of second or third round) in training who accepted a "leader" or a coach or general manager, is a new position elsewhere. In its complaint, the NHLPA argued that this had a major impact on the number of teams that could make unwanted offers (offer sheet) to free agents.
More Lavoie (on the agreement between NHL and NHLPA): Here it is: during the period of free agents this year and in 2017, Columbus, Edmonton, Toronto and San Jose will therefore be (like 26 other teams) make offers to free agents, even if they don't have more choices for second or third round. They may do so by giving them choice next draft or by giving them choice of second or third round they acquired through such transaction or a choice of the following year.