im sure they showed in the same episode (or previous ep) that they were accurate with their shooting on the dragon (albeit a bigger target) and they hit the rider (daemon) whilst sitting on the dragon which drove him away.
If you have about 500 shots at one, the chances of them all missing by cm's is near impossible.
The thing with the scene was it just felt out of place with everything else (or not even that, it felt out of place given the information we had up until that where the crabfeeder and his people managed to keep them out for three years).
This is scorpions > dragons > scorpions all over again. Just be consistent, it shouldn't be hard for a good writer. I liked the episode as a whole, and the show so far, but it's a slightly bad sign that they couldn't find a better way to resolve the plot point.