I'm not sure exactly what the reason was, they threw out a couple different reasons. Initially it appeared they thought the tuner caused some unexpected problems, but now they are saying otherwise...?
Original Reason -
There was a sticky post on the EA forums for a few days that has since been deleted or moved saying they found out the tuner caused an unexpected problem with a few things including Tyson and Ali's ratings (Ali's power dropped to 84, Tyson's power dropped to 93 and speed to 87) and they would be deploying another patch specifically to fix this.
Current Explanation -
It’s amazing to see now over 500,000 online games being played each day. You have probably noticed that we updated the tuner file last night to the original settings. With the tuner set functionality test completed for the demo and the online portion of the demo ending Feb 17th, we are now focusing on ensuring functionality for the full game. Further for those who noticed different boxer ratings online and offline, this is a display issue in the demo and does not affect gameplay. Thanks for your suggestions and feedback.
http://forum.ea.com/eaforum/posts/li...7CBF2A66FCDCD8
If I had to guess, the tuner was rolled out for 2 reasons, #1 to test the functionality of the tuner in a live environment and #2 to appease the vocal critics of the game that were spamming the EA Forums night and day complaining about how much they hated the demo.