The USPS is not subject to anti-trust laws.

The rest of your statements include the word "bought". There is a difference.

Also, those deals(Challenge, Menlo, etc.) are all being looked at as a whole in this deal. UPS has quitely and methodically been transforming itself into a monopoly. When you put all these deals together it is huge. Imagine if Kroger bought Piggly Wiggly a few years ago, then moved and bought a portion of Publix, then a few years later bought Winn Dixie. Individually these seem harmless but put together, they are huge and harmful to the American consumer. Anti-trust/collusion laws are very complicated, most of which I don't even attempt to understand but I do understand what they are looking at here and why.