I'm not sure it is. First off, the UK isn't a signatory to Schengen. There are certainly plenty major airports in the Schengen zone (AMS, FRA, MUC, BCN, VIE, to name but a few) with no such restriction.

There have been some big changes to Schengen airports to separate the Schengen and non-Schengen zones, however. This may involve building new piers or terminals (or expanding domestic ones), building new floors on old departure areas (sharing the gates), or having passport control at the gate, manned according to the origin/destination of the flight. Transfer between the two zones requires passport control.