That's nice & humble of you, but it's not precisely true. There are very few requirements for taking a random course or two (or 20) at HES, but to be admitted to a degree program at HES is not so easy. You have to take a certain number of HES courses and earn a high GPA in them to be admitted to a degree program. Then, of course, you have to pass the full slate of courses to earn your Bachelor's degree.
Some people also seem to be under the impression that HES courses aren't as difficult as regular Harvard College courses. If anything, I'd say there's far more grade inflation in Harvard College than in HES, i.e., it's probably easier to make an A in a lot of (though obviously not all) regular Harvard courses than it is in Extension courses.