I do not believe there is any purposeful or malicious intent from Microsoft. But that doesn't mean they cannot or should not be held responsible for the data that they host and serve.

Doesn't reddit serve thumbnails?

Wave after wave of recent subreddit bannings due to reddit now believing they will be directly responsible for data they are helping to serve should make one stop to reflect about how exactly such a law would be enforced.

Fine, but responsible in what way? Not like they did it on purpose. Their bots found the images, Bing just forgot to exclude them. Now they're work a lot more to exclude them.

I think Microsoft is being purposefuly laxer than other engines. Their main intend must be to have more ‘regular’ porn, but child porn will also fall in the basket.

What’s the gain? relevance among porn users.

Perhaps we can compare this to fishing. Nobody catches dolphins on purpose, but a lot of fishers will use wide nets well knowing that some dolphins will be trapped in.

They may not have intent, but they are clearly acting negligently.

By a legal definition of negligence?

If there wasn't a gain, why would they process or index them in the first place? If you don't want to be responsible for the data you process, don't process it.

Because classifying huge amounts of content is highly non-trivial. It's not like there are a million employees sifting through every spidered resource and manually deciding whether it should be indexed.