A few reasons. The Accord has a different engine as the Civic Si's K24Z7, with the same compression ratio and displacement but with less power.

First, the Accord engine is one of the new EarthDreams engines, which has direct injection (directly into the cylinder) rather than fuel injectors on the intake manifold that mix air and fuel in the runners. This probably has something to do with it since the fuel travels for less time before igniting. This means there is less opportunity for the fuel to detonate ahead of time.

Second, I see that the Accord engine makes 10-15 HP less, depending on the engine code (regular models make 185 HP/181 TQ, sport has 190 HP/185 TQ). This is probably from the engines being more conservatively by the ECU compared to the Civic Si, and also from different (more conservative) cams. Different cams have different timing, meaning that the more conservative cams are less prone to causing pinging.



Basically, they are not the same engine and while they have the same displacement 2.4L and compression ration 11.1:1, the Civic Si has more aggressive cams, a more aggressive tune, and does not have direct injection. These 3 things make the Civic Si require a more stable fuel.

The accord sport is not meant to be the high power Accord, so it is marketed towards people who don't want to use premium fuel (most common drivers). That is why it is conservatively tuned and has conservative cams. The V6 Accord serves the role of a trim level for people who want to pay for more power (although I don't think it needs premium either... its higher displacement makes up for the need of more aggressive cams and tuning paired with higher octane fuel).