This was a TA (T->I) GUF though which is why I was surprised - because I'd followed your logic too in my head.

I agree, the RDUG tag seems pointless, especially if there's nothing for a plain miles ticket. However, isn't it the RDUG tag which ensures things post correctly with 'base' miles/points? This is the only thing which differentiates a U class from MFU vs. straight U class.