National has never held $300 for me at my local airport or "foreign" airport. They seem to always put a hold of the full rental amount upfront, and then make the charge on return. Enterprise however, puts a $300 hold and then deducts my rental from the $300 hold when I return the car.

I think they are supposed to always ask for your credit card and DL for verification purposes. I feel like the credit card is sometimes asked more so to make sure you are taking out a car with adequate coverage