...I don't know too many that would:
a) know of Denon;
b) spend US$1200+ for a Rx;
c) be comfortable doing their own firmware updates (if they even care); and
d) not have broadband...
Wii requires broadband ethernet because there's no other alternative. E.g, on-line game play requires a live connection. You can't do that by downloading firmware to a USB thumb drive.
By contrast updating the Denon 3808/4808 firmware is a maintenance and support issue. There are many technical alternatives to broadband ethernet for delivering that. Most other consumer devices -- even very sophisticated, expensive ones -- allow firmware updates via non-broadband methods. My Nikon D300 DSLR costs more than my AVR-3808CI, and is arguably just as complex to use. Nikon could have reasoned the technical user base buying that camera has broadband, so we'll require that for firmware updates. They didn't -- firmware updates are done via a downloaded file.
The issue isn't "Joe Six Pack can't afford broadband, but Denon's not interested in those customers". Rather a large % of potential customers simply cannot get broadband. It's not a matter of affordability or customer sophistication. This may be difficult for urbanites to understand, and especially younger people who grew up viewing broadband as a required utility like water and power. But broadband is just not available for a large % of the U.S. population.
