That's true. The stations KMPX added are 100 kW FM stations I can receive easily, even in "HD" (Hybrid digital). It's not as if lots of folks need KMPX's help to hear them clearly.
I suspect they're just going for convenience. TV's and radios are (usually) different boxes. When you're surfing TV channels you usually don't even think about surfing radio channels too, but if the radio stations are also on TV you may stop to listen (or even switch to the radio, if your radio has better speakers). Both DirecTV and Dish Network do something similar by including a selection of Sirius/XM radio stations in their "TV" lineups. That's not a perfect analogy since Sirius/XM is a subscription-only service, but the point is, my wife does occasionally listen to them.
Audio uses less bandwidth than even SD video, so it costs a TV station little (in either money or video quality) to air audio channels. If CBS didn't have a blanket "no TV subchannels" policy, I could imagine KTVT/11 and KTXA/21 adding KRLD-AM and -FM, respectively, as audio-only subchannels. (KRLD-FM already simulcasts -AM as an HD subchannel.)