To my knowledge, actually, by default, Comcast solely provides IPv6 by default... but then if you plug in a device that requires (or is configured to require) IPv4, it'll give you an IPv4 address. During the transition, I'd occasionally find weird things would spontaneously break on consumer PCs, like old Office Click-to-Run versions which didn't support IPv6, and then discover the user no longer had an IPv4 address.
Usually happens if the customer's computers connect to the Comcast gateway directly. If they have their own router, it usually gets an IPv4 address.
Comcast is dual stack, and will hand out IPv6 and IPv4. There are times when their IPv4 DHCP server is slow or seems to be out to lunch though, and during that time you might get IPv6 only.
That is probably just a side benefit. Your two largest ISPs pushing IPv6 are Verizon and Comcast, because they're also (including wireline and mobile) the largest ISPs. The number of IPv4 addresses they'd need to meet their customers needs would be astronomical if they didn't find any excuse to go IPv6 only where possible.
Usually happens if the customer's computers connect to the Comcast gateway directly. If they have their own router, it usually gets an IPv4 address.