Ah, good idea. EWR, ORD and IAD. All three had only mixed lines that I saw.

It was curious to me because separate lines can be quite a lot faster. US citizens and green card holders have guaranteed entry and are processed differently and more quickly than everyone else. Separate lines make a lot of sense.

Except if a different motive were in play. Not having a separate line for US citizens might make sense if you were trying to incentivize people to enroll in Global Entry.

On a side note, I never use those kiosks, I just walk right by them and go straight to an immigration officer with my passport and a paper customs form. I haven't looked at the law, but I'm assuming the kiosks are not found in law, so, at least for a citizen, there is no legal requirement to interact with the kiosks for entry, so they're optional. Based on my observations, they are slower to use than the paper form.