I agree but that's not what's happening here. They are just directly passing on the cost to provide service - the customers are paying Hetzner not the ISP (unless I'm reading wrong - but I did double check).
In this situation, Core-Backbone pays for peering with DTAG and Hetzner pays them to transport their data. I'm not seeing how that is an ISP billing another ISPs customer? Hetzner is paying Core-Backbone, Core-Backbone is paying DTAG. Same as Netflix paying Cogent to transport their data.
I'm primarily thinking of cases here like consumer ISP:s wanting to charge Netflix directly. This particular case might be quite different from what I had in mind.
In this situation, Core-Backbone pays for peering with DTAG and Hetzner pays them to transport their data. I'm not seeing how that is an ISP billing another ISPs customer? Hetzner is paying Core-Backbone, Core-Backbone is paying DTAG. Same as Netflix paying Cogent to transport their data.