The molestation was years ago and Josh had already repented and been forgiven. As far as they were concerned, it was over. Plus, it was partially the victims' fault for not being modest enough, Josh was young, it happens in a lot of families, the girls don't even remember it, and they dealt with it. The Ashley Madison scandal was worse because Josh was an adult and he sought out sin. There wasn't anyone else to blame but Josh, which made it even worse because then it reflected on the whole Duggar brand.
To Evangelical Christians, repenting and being forgiven means that whatever you have done has been washed away. After all, if God/Jesus has forgiven him, who are we to continue to condemn him? Even now, I'm sure they feel like Josh has been to Jesus Jail, his wife has forgiven him, so why can't he slide back into his place on TLC?