In the way you suggest, the government does trust you.
However, there are issues beyond trust. These are simple reality. Human beings eat, and a certain time later, they excrete. In the absence of any public sanitation measures, the health risks follow an exponential curve. During the Katrina aftermath I calculated it would require 300 portable toilets to service 30,000 people at the Superdome.
Per day.
Several million people is a whole different order of magnitude. Add that to a shortage of drinking water, and the risks increase yet again. It isn't a matter of trust, it is the logistics of literally tons of human waste that needs safe disposal. If 5 million people each generate a quarter pound of waste per day, that comes out to 625 tons, or better put, 15 or 16 tractor trailer loads per day. It doesn't just vanish, not without impact.
The other simple reality is that there are only so many freeway lanes, which each only hold so many cars.
In short, moving a couple million people back into the city requires either a little advance planning, to check the operation of basic services and to allow for orderly transit, or else it requires outright negligence if none of this is done.
No government worth having would choose the second course. They know a certain percentage will make their own decisions. You won't see them prosecuting the blockade runners after the fact, though they may chase a few down to make a point. Their big concern is for the bulk of the population and the policies they are following generally follw the needs of a group that size.
When that doesn't happen, then there should be and almost always is, an outcry and resultant change.
Now you have gone and done it...
Look, we moved somewhere around 3 million people out. Remember the rows of porta-potties along the evac routes? 36 hours. Yes, people had to relieve themselves. Remember the human excrement 3 feet deep on I-45?
The porta-potties did not happen. Neither did we cr@p all over the interstate and wallow in it. Jeez-Louise man. This is exactly why Texans do NOT want to hang out in a FEMA-run shelter. They just want to get back as close as they can to pitch in and fix stuff. Y'all will thank us for conserving money and freeing up federal resources sooner than later.
Most displaced Texans *are* staying put, though - that's the point.
http://www.freerepublic.com/focus/f-news/1490933/posts
Looks like the surge wasn't quite as gentle as we ultimately believed.