Understood, but if they can tune a radio, they can tune through HD channels. It's just like tuning through the FM band from channel to channel. But that's if it's an actual radio, and most cars have flatscreen, icon based soundsystems.
And I think most car dashscreens are icon based, like your phone's screen is. I know that the Pandora app on my phone is icon based. You hit the icon for the music channel you want. Or scroll through the screen full of icons. One of the issues with car systems is that you can't take your eyes off the road when driving, so unless you're looking for a wreck, you're not going to be scrolling through icons.
I'm sure that's at least one of the reasons people don't tune around through the HD2's and 3's. And if they are unaware what is offered on the HD2's and 3's (the only way to know is research them online -- and many FM websites don't even have info on the HD2's and 3's) is if you actually tune through the channels to see what's offered.
I'm not sure about the FM station apps. I know a lot of stations have apps. I've just seen the station websites and web players. The few that have HD2's they don't seem to have them accessible. I would think the station's app screen isn't much different.
And I don't think your average radio listeners anymore tune around to see what's on the FM band.... it's use your favorite preset or bluetooth your phone's music internet stream.