He already molested several of his own sisters when they were children and she knows that. Do you really think she was only monitoring for adult porn and child porn wasn’t a concern for her at all?
Unfortunately the religious system this family lives within doesn't differentiate the two. To them, any sexual sin is a sexual sin and they're all equally bad - but the remedy for them isn't therapy, jail or community service, it's prayer, repenting before god and accepting god's forgiveness (which, of course, no other person can check to make sure has actually been granted), and confessing to and being forgiven by the community (said forgiveness is not optional). And that gets even more troubling when you realise that 'sexual sin' in their community covers everything from looking at someone else's cleavage for a few seconds too long or teenage self-exploration in the shower to rape, child abuse and spousal abuse.
Worse yet, the system they follow teaches that victims or sexual abuse or assault are, somehow, any way you can screw reality up in a batshit little knot to make it so, at fault for being abused or assaulted - and even if there's no way anyone can make it out to be their fault, taught that god allowed it to happen to make them spiritually stronger. There are images of the specific teaching material posted in various places online; if I can track one of them down I'll come back and post a link.
It wasn't only Josh Duggar who had to confess his sins before his community after he abused his sisters as a teenager. His sisters had to as well.
By the way, I thought people might find
this post interesting - it's by an attorney with education in criminal law talking about why the judge pretty much had no option other than to release Duggar on bail at this point.
ETA:
There are many reasons to be

about the judge's decision to grant bail to Josh, but I didn't know until today that in the family that is responsible for him, the wife teaches piano. To children. So the judge released him to live in a house that has even more kids in it than most households would.
IIRC it was the family's adult daughter who teaches piano, not the wife (who sounded all kinds of unwilling to have this man in her home), and they agreed that she would find somewhere else to teach for the duration of Duggar's stay.