Alma's people came together for a religious cause, and their entire community was based on the commonality of their religious convictions. They were led out of Helam by a prophet, and by what may easily have been seen as a miracle. It is therefore of no surprise that when imminent danger was over they should thank God for their deliverance. What is interesting is that there is no mentioned sacrifice. When Lehi's sons are delivered out of the hands of Laban, Lehi explicitly sacrifices (1 Nephi 5:9). Here there is no such mention.
We do not know whether or not the principles of sacrifice were becoming altered for the Nephites by this time, or whether this is simply something that Mormon neglects to mentions.