I think you will find it was an easy way for Ford to differentiate the model range. GLI and Futura got the amber, high end models right through to LTD got clear. If you look at the series one AU's you will see they continued the theme. Forte and Futura got amber indicators and Fairmonts and Fairmont Ghias etc once again got clear.
Things changed again with series 2 AU when the base models were dressed up a bit to try and make more sales.
In the BA all models, all the cars get the clear taillights, so they used a chrome bezel around the Futura and Fairmont headlights this time to help differentiate them from the sportier models with black surrounds and presumably to make them look classier (or more conservative) in the eyes of their intended customer. This time though Ford gave the base model the black finish as well as the Fairmont Ghia, to help tie them with the XR's. Same sort of tricks used in the Fairlaine and LTD range. If you read their press gumf you will find it was intentional, to help make XT owners etc feel they had a link to the sports models in the range. Even flint mentions the tie in with the XT by calling it a cousin of the racing car.
Somehow I have trouble imagining my XR8 has anything more then a passing resemblence to a race car, let alone an XT. Still its the idea that counts.