Yes, one particular plotline which can explain why docsis systems do this is that standard residential customers are provisioned using giant broadcast domains directly on the cable, with DHCP config. Obviously it's more complicated because it's docsis, but lemme handwave and say that this is the gist of it. Because you're dealing with giant broadcast domains, you assign IP address blocks to individual CMTSs and your customers are assigned out of those ranges.
Assigning ipv6 in this context is really simple: it's part of the baseline DOCSIS3.0 standard and is supported incredibly well by all parts of the network.
Static addresses don't fit into this paradigm because you if you configure your static customers from a single broadcast domain, then they are glued to a particular CMTS and can't be moved from that CMTS unless you renumber them.
This doesn't work in practice because if you want to grow your network, you probably want to be able to move around chunks of your cable network from one CMTS to another in order to balance out your traffic. Or you might want to split a bunch of cable nodes from one CMTS to multiple, according as your traffic outgrew the capabilities of a single CMTS (a node in this context is a small chunk of a cable network).
One way of getting around this mess is to backhaul all your static customer interfaces using mpls l2vpn PWHE up to a L3 box which just handles static IP addresses. You configure the customer's static default gateway IP address on an interface on this head-end router, and the customer's cable modem will have a virtual connection directly to that interface. The thing is, this virtual interface termination system might or might not be tied into the entire ipv6 provisioning system. If it isn't, you're SoL. So even if dirt-cheap residential customers can get ipv6 very easily, it's different by virtue of the fact that you're using static IP addresses, because they're a headache for cable operators.
aha! makes sense. i'll settle for dynamic. if i need static internally, i can always do nat66 :)/2 i do not want to play how hard can we make ipv6 deployment; just want to enable it on a five-segment office lan. but i am beginning to see that there may be a reason i am having problems getting past an account rep. randy