The name of this forum is 'LinuxQuestions'. Rightly or not most people refer to some distribution they are running as Linux, not Gnu/Linux. Linux in this sense refers to the OS, which includes the kernel and supporting software. I understand Stallman's POV, but unfortunately or not the term 'Linux' almost universally refers to the whole shebang and not just the kernel. No one in their right mind would refuse to recognize Stallmaan's, and Gnu software's, contribution to what is popularly called the Linux OS.

I did mention 'kernel events' but I did not mean to imply that a library is something that only runs in kernel-space. I well understand that creating a library to handle kernel events can not slow down normal kernel processing just to provide callbacks for a user-space program. I was just surprised that most libraries created to provice some way of notifying other programs/libraries of kernel events are not based on callbacks but low-level monitoring. I view callbacks much more programmer friendly and a widely accepted practice for event-driven programming from my own experience.

What programs/libraries are you talking about?

I still think that is the normal way. introducing kernel-level callbacks will add extra load to the kernel itself, so that won't work.
you cannot run user-space programs by these callbacks, you need to use some tricks to start a user-space program. That's why was created that monitoring (user-space) app.