You would think that, unfortunately it isn't true. Usually those ISP provided combination routers are completely outsourced - as in their hardware and firmware are both made by a chinese OEM overseas.
Believe me when I say there is nothing worse than the firmware on those chinese routers. They'll grab a bunch of open source software, reskin it, plug the chipset's reference driver into the OSS, then do a basic sanity test and ship it. You get what you pay for, and the ISPs want to pay as little as humanly possible.
I don't know what they do about the support. I remember one time we brought in an ISP's combination modem/router for performance tests against our device lineup, and it would crash if you ever tried to pull more than 40mbit/s through its ethernet ports.