OP isn't true but partially true.
Most common and ubiquitous FTTH service (Flets) only provides fiber but no internet connectivity so customer also need to sign up ISP to connect internet. Even without ISP, Flets provides IPv6 address and network service without internet connectivity, but can use VoIP, IPTV, VPN, and other non-internet services.
But generally no individuals sign up only Flets without ISP. Nowadays it's bundled. Most ISP provides IPv4 connectivity via PPPoE. Many ISP also provide IPv6 connectivity, and IPv4 connectivity via v4-over-v6 technology like DS-lite, MAP-E. AFAIK no consumer ISP charges v4-over-v6 service fee if it provides v6 connectivity.
The weird point in Japan is that sometimes said that "IPv4 connection is VERY slow!" because Flets don't want to invest old PPPoE infrastructure much. Instead, Flets and ISP recommend v4-over-v6 service for IPv4 connectivity because they invest it. As a side effect (maybe not side effect), ISP could save usage of global IP addresses.
Still there are some homes use slow v4 and fast v6 connection, due to equipment doesn't support v4-over-v6 or need a global IPv4. So I recommend content providers to enable IPv6 connectivity, especially for CDN.