And in case you don't know, Schengen passengers are not treated the same as International passengers.
Yes they are. When you arrive in the UK (something I did on Thursday, when was your last visit to Blighty?) - there are indeed different arrivals channels, but there is no distinction based on Schengen.
The first separation is "domestic" arrivals, and this includes the UK, IOM, CI and also Ireland. However, arrivals from IOM & CI are subject to customs controls. No passport control is needed for these arrivals.
All other arrivals are bunched into the same area, but then you sometimes get separate channels, based on the passports
held by the pax. This distinction is between the European Economic Area (EEA) + Switzerland and others, but it does not correspond
directly to Schengen.
Although it includes
all of the Schengen area, it also includes UK passport holders, together with passport holders from Ireland (who might not be arriving from the Republic), Bulgaria, Cyprus, Romania and (from 2013) Croatia.
Therefore, although the two are related, it would be quite wrong to mix them into one and have Schengen and "international" gates, as is the case in most continental European airports.
Even if people were making Schengen to Schengen transfers (eg CPH to FBI to BCN), they should not need to go through passport control any more than someone transferring from the USA to the Middle East - providing the terminal was designed to handle this, and everything was kept airside.
However, by the time FBI would ever get built, we might be all using RFID implants, so the concept of nationality will eventually become far less relevant in airport management. The pre-crimes dept will be far more interested in behaviour patterns whilst the commercial forces will want to suck you through whatever buying opportunities they can.