I think each situation varies by airport. It might be helpful to mention which airports you've been through. At Seattle-Tacoma, the last time I went through, they had separate lines labeled as "US Citizens and Permanent Resident" and "All Others, including Visa Waiver Program." The first line went to rows of kiosks and then, for LPRs, to a smaller collection of agents in booths for the reentry interview and biometrics collection. (US citizens could take the kiosk receipt and show it to leave.) The second line went only to a larger collection of agents in booths, no kiosks. There was also a third bypass line for NEXUS, Global Entry, and SENTRI holders that went to a different set of kiosks.