When you choose to Share a Course, a public URL is given so you can send it to anyone you want or you can use the invite button to directly send them an email.
In this way, the users with the link can access the course's content without even having an account in your portal. So, the Courses will be completed anonymously and no reports will be generated for them.
If you want to disable this option you have to click on the Unshare button (1).
Note: Courses with prices can not be shared to users.