[majewsky]

I have the same situation with Vodafone Cable in Germany. I was told it is because of 6-to-4 bridging. A customer on IPv6 must be able to bridge to IPv4 to reach IPv4-only servers, but a customer on IPv4 will probably never encounter IPv6-only servers (except when using an IPv6 testing tool).

I'm not a networking expert, but AFAIK there's nothing stopping them from making their 6-to-4 bridging available to customers with a custom router. However, getting that stuff up and running correctly can be tricky, so it would involve a lot of support burden on the ISP's side. Hence why they don't offer that, they just drop those customers down to a configuration where the customer gets to use well-known protocols only (IPv4 and DHCPv4 over Ethernet).