The astonishment may be credited to a difference in the nature of prayer between the Nephites and Zoramites. If Alma and his brethren are astonished that all are giving the same prayer, that indicates that their expectation of prayer is that it ought not be so formal. However, their astonishment might also be due to the anti-Nephite content of the prayer, which combined with the religious difference, should have been quite disturbing to these emissaries.
What we should not forget as we read this passage is that the prayer had an important function for the Zoramites. While we might not see this as the proper form of prayer, we must still recognize that the public repetition of these phrases functioned to confirm a unity of worldview among the Zoramites.