And they are right. They are many ways to cheat. And there are no ways to prove you didn't.
The system on Coursera (the one I tried) is actually very "relax". You have weeks to do your weekly tests, and weeks to do the final exam. You can edit as much as you want your answers, and you can re-do a test which is usually the same questions but with different answers (and also, when every time you submit a test they tell you what you could have done wrong).
If you wanted to give real credits for that, then you'd have to put a real limit of time to those tests first.
But I like the way it is. We don't learn something for credit anymore. I hate going to a test because that's not what I want to do when I go to university. If you don't get real credit when you pass a course on Coursera, then why would you cheat? You can just learn for the pleasure of learning.