Google removed a search result, why ?

Did you read the message?

Google removes things due to being asked or being told.

They don't do this to normal search results, which is what
you are probably asking about.

Those messages appear when one searches for images,
movies, etc.

Of course they won't tell you when they have de-indexed a
gazillion sites.

It's not the same thing. Has nothing to do with SEO. Those
messages are due to people, companies, or organizations who
have brought it to google's attention that they should not
be showing certain content. Sometimes google does it on
it's own, sometimes when a court orders them. Or their
lawyers advise them to.

Not the same as a simple, normal de-index.

Sometimes images or content is so vile or illegal...

I presume google puts that message there to remind people
that they do not do it as a rule, but only when "forced" to.

Again, that's different than them de-indexing 100 of Joe
Blow's sites due to any number of webmaster violations.

Paul