But you are right, in a way the FTC is appealing their own decision [1]. US politics can be quite mad at times.
[1] https://www.ftc.gov/news-events/news/press-releases/2012/08/...
Efficiency? The people at the FTC reviewing mergers can't be experts of every corner of the economy, but if they catch an illegal merger during the approval process it can be blocked early without having to go to court.
An illegal merger is illegal no matter what. It's the corporation's responsibility to not break the law.
> The point is about why it's acceptable for the FTC to approve something, and then years later come back and change the decision.
I addressed that in my comment (it was the entire point of my comment, actually)
No question about the truth of that statement.
However, though the FTC approved the acquisition 10 years ago, the current FTC commissioners have evidently concluded that in the interim things have changed. Whether the court agrees with the FTC's logic remains to be seen.