This way, the 'instructors' who gives out free courses will gain in popularity, udemy will encourage new instructors to make courses because they thought other instructors are earning money. I mean 20,000 enrollment x $15 = money. In reality, it's way less than that number.
For example, if we each sign up for 5 courses, we're going to 'keep it' and maybe watch like 1 hour of 1 of the course. They, in return, udemy becomes more popular, and instructor will get an extra number in that enrollment with no loss on their part.