That's not surprising at all, and would be required even if Buran was developed completely independently to STS. Without that manoeuvre the engines would thrust

That's not surprising at all, and would be required even if Buran was developed completely independently to STS. Without that manoeuvre the engines would thrust off-axis due to the side-mounted payload. It's easier to compensate for off-axis pitch than yaw when you have gravity to account for. For Soyuz launches on the other hand they physically rotate the launch pad.