I think the point is that if such a system were adapted online, it would be prone to abuse. It's easier to "check out" content digitally then it is through a traditional library.
The solution they've adopted in Sweden is that each library card is only allowed to make n digital checkouts a month for some low value of n.
Although the problem they where trying to solve wasn't so much abuse as the fact that the service was far more popular than they'd anticipated and legitimate use was high enough to seriously risk their budget.
