It doesn't seem that weird to me. The line is active so you pay. If you don't want to pay don't pick up the phone.

You often don't know who is calling you until you pick up the phone, so you can't make a judgement about whether it's worth paying for the call until you've already started paying.

That, combined with the fact that it appears to be unique to just the US & Canada (maybe NANP in general), makes it bizarre to me.