They haven't had it in 20 years, it used to be on everything. WXYZ was the same way with the ABC logo, they skipped out on it until 2008. They had to be forced to add it. Network logos on local bugs is dumb anyways, 70 years later I think most people can find the channels, no need to parade it out there anymore. And since it's a newer thing, no need to have started.
Another reason is that people in this market identify the network as part of the channel and not the other way around. So NBC is second to WDIV in viewers thinking, same with ABC being second to WXYZ. We've just never done in that way, same with time and temp, outside mornings neither station uses them unless it's extreme heat or cold. Detroit is a very unique market and it doesn't seem to run like most others. It's a #5 market that dropped to 11, so it' doesn't quite line-up with it's rank.