According to the New York Times which is embedded with the military and therefore in position to know the units taking part in the taking of Marja did not cordon off the town in any meaningful way. Their map below clearly shows this:
So not only did the Americans announce their offensive a month in advance, but they also intentionally permitted the enemy an escape route instead off boxing it in. This is consistent with the reports that of the estimated four hundred guerrillas in the area at the launch of the offensive up to one half had melted away since.
This is smart on the part of the attackers, there is no use in taking needless casualties which would certainly be higher if the enemy was pushed into a corner. But if they were truly smart they would not have launched this offensive in the first place. With it they are taking on a commitment they can not fulfil. They can take the town with military force, but they can not use it to create conditions where if they leave, the town will not simply revert to its condition before the offensive and sever its links with the present government in Kabul. So they are taking on a commitment to garrison the area indefinitely - something which is clearly not feasible.
Just like Americans have the ability to take things slowly on the tactical level - something which they are making apparent by advancing at a glacial pace in order to keep their casualties low. So can the opposition afford to take things slowly on the strategic level and wait them out until such a time as they vacate the town.
After all, they live there. They are not going anywhere.