Get away with collusion? Well, let's look at it. Collusion is any kind of agreement or encouragement, either overtly or covertly, between two or more legally distinct entities competing in the same market (in this case, owners) designed to suppress wages. A salary cap, luxury tax, or any other mechanism you can think of designed to suppress wages is technically "collusion". Essentially, it is the "conspiracy" charge for violating the Sherman Act.
HOWEVER, if that mechanism is approved by labor through the collective bargaining agreement, the "collusion" is made legal. In other words, the legal system is saying that if you want to do something like collusion, you've got to get your labor to agree to it. Otherwise, we'll protect labor and make it illegal for you to just artificially deflate wages.
So, let's presume that a luxury tax has been approved through the collective bargaining process. Any time an owner willingly and without encouragement from another owner decides to stay under the luxury tax threshhold, then there is no collusion. Again, HOWEVER, if after there is a luxury tax, two or more owners get together and agree (overtly or covertly) to not go over the luxury tax no matter what, then you have collusion. Basically, once there is a luxury tax (if there is one), each owner is obligated to decide for himself and himself only whether or not to go above or below the threshhold.
I hope that makes sense.