Well, many isp-born devices are provided with that info by the ISP at connect time, so the hope has always been that during configuration they'd just pass (for example) "bandwidth 10mbit docsis" to the network setup routine. DSL modems also typically get this info at startup.
I imagine the main thing that's holding back ISPs from using it on their consumer routers is the (minimal) configuration the user has to do to set to upload bandwidth limit. If ISP routers were also using decent queuing algorithms and not buffering like crazy then that wouldn't be necessary of course, but it doesn't seem like that's going to change any time soon.