There are some important facts to be discovered in this verse. The first is that Ammon and Limhi consulted with the people. This tells us first that it was probably not the normal form of government, else it would not be necessary to mention. In the size of the city under Noah (as well as Noah's probable proclivities) there would not have been a consultation with the populace on any matter. The king would decide and declare. This occasion is different in that the entire population will be effected by the resulting answer. In fact, it is quite likely that the ultimate solution was known beforehand, and the meeting was held to allow the people the opportunity to participate in making a decision that would require them to uproot themselves from their homes.
The second important piece of information is that while they were under Lamanite guard, the Lamanites were clearly not in the city itself. Had there been Lamanites present, a large gathering would not only attract attention, but it would be impossible to conceal the contents of such a large meeting. This meeting had to be conducted with the knowledge that no Lamanite would hear the discussion and know of their plans beforehand.