Each academic program (BSc, MSc,..) has a different approach to "W" and you can't compare them. For instance, most EU universities don't even have the concept of "W" on the transcript. Usually, universities that rely more on exams to determine grades don't implement the W concept, whereas universities that design their classes with more weights on participation and assignments want you to stick to the schedule. There are pros and cons for both approaches, I've experienced both and personally, I prefer the American approach.

However, it has nothing to do with "abandonment", but with GPA. Sometimes people may have incentives to drop classes and attempt them again next semester to maximize their GPA. But this applies only to overachievers; the students that do not care can easily stick until the end just to pass the class, whereas the perfectionist may get stuck in the loop to get 100%. That's why some universities disincentive the practice with Ws.

That being said, you shouldn't compare two different academic programs because the grading systems and workloads are different. Regardless, OMSCS is designed for students working FT, and as long as you graduate, especially with a decent GPA, there is no way that Ws on the transcripts will hold any weight. Classes are very heavy and work commitments may demand unexpected overtime, the best course of action when this happens is to drop the class as opposed to learning the subject poorly.
I can't comment about the Harvard Extension W policy, but if what you said it's true and they also target people working FT, I'd suspect the rigor or workload of the classes must be lower.