It is extremely easy to cheat on a lot of online courses. One major flaw is many instructors use a test-bank for all of their exam questions, which makes verbatim Googling for the answer a piece of cake.
There are examples to the contrary though. I just finished "Introduction to Systems Biology" from coursera, and the quizzes and exams where tough. They really required you to have understood the topic and googling didn't help me a lot.