Either they really don't understand IPv6 or they are creating a false scarecity to charge you more money for what is essentially a free and virtually infinite resource to them, probably both.

Or they're running 6rd and giving him a dynamic ipv4?

Not sure what they're running. They do hand me a dynamic IPv4 address, but unless the modem is offline for more than an hour or so, I get the same address.

For whatever reason I get a new IPv6 prefix every time the modem or router reboots.