Because the desire to whitewash Vivienne is rooted in racism
Not necessarily. The desire to whitewash Isabela was driven partly by the difficulty the engine had rendering dark skintones. When I modded her, my first chocie would have been to make her darker (because dark skin was more appropriate given her background), but that didn't work in-engine, so I went the other way.
All changes of appearance are equivalent. I modded Leliana's appearance in DAO, once, because the colours of her portrait were too similar to that Warden's portrait, so I modded her for reasons relating to UI function.
There are any number of reasons why someone might mod the appearance of a character. And I see no meaningful difference between changing skin tone, facial features, hair colour, or giving them wings or fangs. They're all simply cosmetic changes.
Moreover, taking away someone's ability to whitewash her doesn't affect the pre-existing desire to do so. So even if the motive were racist, what is gained from preventing the mod?