It was a parlay. I guess you could argue why Cersei would adhere to the rules of a parlay, but part of her plan seems to be to paint Dany as a crazy foreign usurper, so maybe she felt following the rules makes her look more dignified.
She wouldn't. The scene was created purely for dramatic purposes. Logically it's completely idiotic to line yourselves up in front of weapons that just killed a dragon and took out part of your fleet, parley or not. And it would have been entirely in Cersei's character to just wipe them all out.