While I love a good yarn as much as the next man, the fate of prisoners in ancient times never resulted in a 'prison camp' where all the defeated army was kept together. Too much trouble to maintain them, feed them, etc., not to mention the possibility of an armed uprising.
However, there was a great market for slaves to fill the coffers of the winner (with a little booty for the common soldiers)
On the other hand, some victorius generals simply liked to slaughter the defeated army.
The routing of an entire Roman army is as much a yarn as anything. A more likely scenario was a deal was cut.