The precogs saw Anderton shoot him. They discovered that he went there KNOWING he was going to kill him. The date was in the (relatively) far future.
The precogs see flashes of these future events, and what they saw was Anderton and Crow in the room, with Anderton knowing he was going to kill Crow. They saw the gun firing, and Crow dying.
Thus, the 'premeditated murder'.
As for the second part, it didn't happen because Anderton never left the jail at all (number 2 on that page).
As stated in the Cracked article linked to:
Less obvious is the shiny, happy
ending of the neo-noir Minority
Report, with the three rescued
psychics living out their lives in an
idyllic cabin right out of a Thomas
Kinkade painting. It feels so
out-of-place that it could very well
be a delusion of Tom Cruise's
Xenu-infested skull. Remember,
Cruise's character was briefly
imprisoned in that futuristic iJail
where the prisoners' brains remained
free to dream. It's possible that the
"escape" and heroism was all part of
the fantasy that keeps the prisoners
from escaping (just like the "Zion is
also the Matrix" theory).
After Cruise's character 'escapes' from the jail, the tone of the movie changes significantly (if subtly). In Anderton's mind, the good guys win. The bad guy is faced with his choice, and makes it - PreCrime goes down the toilet as people are faced with the mutability of the future.
Except that it didn't happen. Anderton stayed in prison, never escaping. The bad guys win, and PreCrime expands.