Because there is a population of Burmese Pythons in the everglades. This is why they were able to pass it. If there was a breeding population of Retics out there, they would have also been banned. Count yourself lucky, the people who push these bans also had boa constrictors and, I believe, ball pythons on the list. They've got their foot in the door, so expect them to come back and push for bans on other species in the future.
The ban makes it illegal to import the listed snakes (burms, anacondas, rock pythons) into the US. It also makes it illegal to cross state lines with one of those snakes. Which is just plain stupid, because now when someone moves out of Florida, they can't bring their Burmese with them, so they are forced to break the law, give it away, kill it, or release it...which is what the problem was in the first place. Although most people state that the burms in the everglades are there because they were released as pets...I'm now hearing that there is good evidence pointing towards a breeding facility that was destroyed in a hurricane as the reason they ended up there.
You can legally have a burmese python as long as you purchase it in the state you live in, and don't transport it across state lines.