The league obviously did not want kneeling as they are losing revenue -- so they passed the buck onto the owners, who whether they like or not, are going to pay the fines because they exponentially more invested into players, and by association, their morale, than petty fines. So, again, PLAYERS are not getting fined / suspended for kneeling, the league placates 50% of their viewership and owners have to pay a tad more (its telling that they would rather pay fines than have a 15 yard penalty at the start of the game)
So, if the players union want to strike over something they can still do, and not lose money over (aside of the plethora of minor things they do get fined for like: curfew, dress code, tweets etc) then if i was a player, i'd be pretty pissed that i would be losing massive income over something i can still do and not get fined for.