Players union or not... I don't see how MLB can reasonably fail to punish Astros players. They knew very well they were violating established rules, continued anyway, and tried to cover up the scheme after a game when they thought the scheme was discovered. From page 5 of the report:
"Most of the position players on the 2017 team either received sign information from the banging scheme or participated in the scheme by helping to decode
signs or bang on the trash can. Many of the players who were interviewed admitted that they knew the scheme was wrong because it crossed the line from what the player believed
was fair competition and/or violated MLB rules. Players stated that if Manager A.J. Hinch told them to stop engaging in the conduct, they would have immediately stopped.
Although the Astros’ players did not attempt to hide what they were doing from Hinch or other Astros employees, they were concerned about getting caught by players
from other teams. Several players told my investigators that there was a sense of “panic” in the Astros’ dugout after White Sox pitcher Danny Farquhar appeared to notice the trash can bangs. Before the game ended, a group of Astros players removed the monitor from the wall in the tunnel and hid it in an office. For the Postseason, a portable monitor was set up on a table to replace the monitor that had been affixed to the wall near the dugout. "