I think Sony only allowed RAD because, like the DMM TV app on the Japanese PSN which got PSVR2 support over a year ago, it doesn't require a headset to use and isn't marketed as a VR app, but a streaming video platform like Netflix or YouTube. They're both placed in the "Video and music" category instead of the "VR and experience" category.
Here's what RAD said in their Discord today about why they're the only app that allows sideloading VR videos:
"if we were ONLY that feature, we would NOT be allowed on the PlayStation. We are a media hub that has creators and studios, and also has sideloading."
