That would not catch every public IP address that is actually unused, because it can be attached to an interface and yet not be needed or actually used by any client. But I don't agree with GP that this is an important reason for the price increase. They are increasing prices simply because costs have increased.
Anything that an IP address can be attached to is already accumulating a charge, just by existing and running. EC2, NAT gateway, ELB, etc. What's "actually unused" then? Minimum amount of traffic? I don't think it's in Amazon's purview to make those judgement calls.
What I meant by unused is that there might not be a client that ever connects to that IP address, so the public IP address itself might not be used even if its attached to a resource.
> I don't think it's in Amazon's purview to make those judgement calls.
I already said I don't agree with GP that this is a motive for Amazon.
I guarantee there are a ton of unused IP's just sitting on accounts doing absolutely nothing.