The text of your question:
For example, I see the point at Avast, Avira and the other free AVs. They make a free product, so their paid version also gets attention.
What's the point with the Microsoft's AV solution? I mean.... why are they doing this? Of course, I don't really ask for company / other inner 'secrets', but a logical explanation. (If possible.)
I'd say this runs afoul of the following "What kind of questions should I not ask here?" faq entries
There is no problem in your question -- "You should only ask practical, answerable questions based on actual problems that you face" and "I’m curious if other people feel like I do."
it's almost a rant, though in all fairness I do think you made a decent effort to avoid this -- "it is a rant disguised as a question: '______ sucks, am I right?'
It is off topic. It's not really a technical question. It's more of a business question: "Why would an operating system company want to offer a free anti-virus product for their operating system, with no paid version?"
Bottom line, PLEASE read the faq. It is there for a reason, and you only need to read the first 4-5 paragraphs to get the most important stuff at the top.