[susan53]

I think you'll find that in all of the sentences there is a sense of some completion. Eg As the course progresses ... by the time you understand more the course has already progressed to a certain point; As you pass through customs ... by the time your passport is checked you are already in customs. Thus in a sense the action is "complete".

But you're right that the action is still on-going - it's "complete" but not "finished". And that's why the continuous is also possible. With a sentence like As soon as the rain stops ... there's no alternative because the action is both complete and finished at the time you go out. But think about As you pass through customs ... In fact the sentence is nonsense because it doesn't mean you keep moving - the action is not, strictly speaking, on-going. You go into customs, stop and have your passport checked, and then go out again. So the use of the simple refers to refers to what the sentence really means - ie that you have already arrived in customs - rather than what it actually says - which doesn't stand up to literal analysis.

Combine this with the sense of "every time" which I referred to in explanation 1 and I think this is why, though both continuous and simple are possible, the simple would often be chosen.