Well, let's be honest, here. We don't actually know what happened there. All we know is what Riven knew, and she ran away after pretending to be dead.
Riven simply assumed that her superiors ordered the attack after receiving her unit's SOS. She assumed that they even received it in the first place. For all we know, they never got the message - war is a messy thing, after all. Heck, Singed could have gotten the message and been like, "Pssshhhhh! Screw that! I wanna test out these cool bombs! The more subjects, the better! BOMBS FOR EVERYONE!". And then strapped the messenger to a bomb and sent him flying.
Or, there could have been a really dang good reason why they absolutely had to bomb the area right then and couldn't afford to wait for their men. Like some crazy champion-level dudes were about to be unleashed, or the Ionians were about to bring out some crazy nature magic or something. Ionia's a pretty crazy place, you know?
Just another reason why Riven should have stuck around, rather than running away. She could have figured out what actually happened there, instead of jumping to conclusions. Then, depending on the results, she could have acted accordingly - whether with understanding as to why it had to happen, or with vengeful retribution and bloodshed.