Yes it's among most antivirus packages advertised features. And example from everyones favorite anti virus vendor https://home.mcafee.com/Secure/CloudAV/HowItWorks.html but they all market a similar feature.

As far as I know most antivirus companies have such defaults which the users can somehow turn off. That means they consider that the user is informed and has agreed by using the product with such a setting unchanged.

I think Microsoft for their threat detection software does the same.

So I guess all the antivirus companies from time to time have such "lucky finds" like these that were obviously automatically collected by Kaspersky. Even the "secret" viruses will eventually be detected in the broader areas from time to time.

Yes. All antiviruses do this. It's one of the major streams of malware samples, and for the company I use to work for -- the most important source -- because those are authenticated as being on real customers machines!