If it helps, think of it this way - say someone uploads 10 images for a comic, and all of them are safe. Then image 11 is explicit. What would you do? Retagging all previous pages because of that one page? Of course not. That would be silly. With all the series on this site, some taking years to complete, you’d never ever keep up.
Anyway, the default filter hides explicit images, so anyone seeing an explicit image had to specifically ask to see it.