Why does the system not allow you to get your free 5th card if you don't have enough credits to buy that 5th card?
There are several ways Totem could have implemented this general sort of offer, e.g.
Buy 4 cards separately get the next free or at a reduced price if it is not the cheapest.
Buy 5 cards in a single purchase and pay for all but the cheapest of them
Buy 5 cards separately and then get a refund on the cheapest one
The second of these is how many shops work similar offers, but Totem have chosen to use the third. I suspect the reason is simply because that was the easiest to implement (especially as it is only a ***** variation on previous similar special offers). The first, I think, would be the most complex to implement.
For most people it makes no difference which method is used. For some it means that they need to reload credits slghtly earlier than they otherwise would have needed to but for most of them this would make little difference. For a few, those that can not buy, or do not wish to buy, more credits it does make a difference, but I expect the number of these to be small.