The reason for the 'pillarboxing was because some of the names during the credits (specifiically Dame Judith anderson) reached from nearly side to side in the frame. Because they didn't want (and are legally obliged to not to) cut off anybody's name they used pillarboxing for the credits only.
In the days of crt tvs there was an overscan issue where they had to be surethe manufacturers built-in overscan didn't cut off the picture.
In these days of flat screens without overscan (or where the consumer could adjust their TV to avoid overscan) it was a distracting travesty to ruin the credits by doing that. If you read the reviews at the time the Blu-rays came out---many people noted and lamented the use of pillarboxing.
And it's done in such a clumsy way to boot.
If we ever get a new set of the movies on Blu-ray--hopefully they won't repeat this defect.