Hello sungyle,
This is a user-to-user support forum. We are just other Apple customers like yourself. Apple employees usually do not read or comment here. I guarantee they would never address a delicate subject like this.
Generally I agree with everything you've said. However, I think you are wrong about one important point. This problem has not been in place for a long time. It is relatively new and has been getting worse. At one time, Apple published some half-hearted, difficult-to-follow, manual adware removal instructions but then withdrew those instructions. Apple allows anti malware apps in the Mac App Store that cannot possibly work unless the developers blatantly violate Apple's own rules for the Mac App Store.
Apple does not usually include adware in the anti-malware security logic that it includes inside the operating system. This serves to make people affected by this adware think, falsely, that Apple has no security. That, in turn, makes them vulnerable to a wide variety of scams. We have a rapidly expanding 3rd party Mac security market that sells software that attempts to duplicate functionality that Apple already provides and generally does a poor job against the adware that so many Apple users suffer from. The rules of this forum forbid us from speculating about why Apple is taking this approach. But I can encourage people to send feedback to Apple and tell them directly how they feel about the situation: https://www.apple.com/feedback/