Just had the pleasure of attending the Winter Meetings and got to discuss this matter with folks on both the PA side and MLB side. Ryno basically sums it up above.
While it is easy to see the grievance from the MLBPA side - the Cubs clearly held Bryant back to gain another year of control, there were also folks on the MLB side who felt it was a huge mistake by the Cubs. Their take was that if a player can clearly help your team today, you have to use him today - regardless of the free agency clock. If the Cubs had missed the playoffs by a game or two, perhaps Bryant's bat gives them another win or two early in the season? You can't sweat 2022 when you are trying to win in 2016, and games count the same in April and September. And as noted above it also doesn't set a good precedent with a key component of your teams future when it comes time to negotiate his mega deal in a few years, and perhaps steers him out the door.
MLBPA emphasized that while they would likely lose the grievance - again, the rules are clearly spelled out in the collective bargaining agreement - this was a necessary step for the union members as it sets precedent for the next round of negotiations next year.