By decertifying, the players are then free to sue on antitrust grounds with the possibilty of treble damages if they win- that's a huge hammer for the players and could result in damages in the billions. Without a union to bless the draft and some sort of salary structure etc the owners are legally acting in a non-competitive manner and are thus open to an antitrust lawsuit.
Of course the numbers are different, but what the owners proposed was about a 20% DECREASE in salaries for an 18 game season. That's about the same as say, you make 50,000/year for a 40 hour workweek and your boss tells you that starting next week you'll be working 45 hours for $40,000/year.