I think, I can understand why, I can not reference any .Net Framework assemblies to any WinRT projects - targeted to .Net Core. Portability and so on... bla bla bla...
But I can not understand, why I can not do it in vice versa!
For example, when I want to add some Windows Store assembly reference to regular .Net Framework assembly, I get error. Is it any logical reason why it is impossible?
Yes, there is a perfectly logical explanation for this: .NET for Windows Store apps (or .NET Core) is not a subset of .NET Framework (or its Client profile). It's a completely separate platform, just like Silverlight or Windows Phone. In fact, it is strongly based on an older version of Silverlight.
The only way to share a class library between a Windows Store app and a regular Windows (.NET) application is to create a portable class library. By selecting the target platforms for it, you'll get a specific subset of APIs that are available on all selected platforms. You'll be able to reference such a library in any project using one of selected supported platforms.