There is no twin-only mentality; the restriction on flights over a hostile environment (and specifically a congested hostile environment) arises from compliance with the ICAO Annexes - you know, the Chicago Convention to which all States agree to comply:
Specifically:
ICAO Annex 2 - Rules of the Air; and Annex 6 - Operation of Aircraft.
Respectively, the need to 'land clear' (protection of third parties) and the requirement for a 'safe-forced-landing' (protection of crew and passengers).
There is also linkage to the the certification principle that no hazardous outcome should be permitted with an event which has a classification of 'Reasonably Probable' (which is based upon a probability on the order of between 1:1,000 and 1:100,000). Because engines tend to fail within that probability spectrum, ICAO SARPs attempt to keep the helicopter away from an area where the effect of the failure could be hazardous.
Before anyone is tempted to point out that there are other failures (including Human Factors) that have a similar, or worse, spectrum, the answer was as specific as the question.
Jim