The twin IFR requirement in Europe is an JAR-OPS requirement so all commercial passenger operations (public transport) in JAA land is twin if IFR and this of course also includes fixed wing.
However JAR-FCL only specifies a IFR approved helicopter so single engine helicopters can be used for training, just like in the fixed wing world.
But the problem that makes it all so expensive is that the manufacturers have only, until now, got turbine aircraft approved for IFR.
I feel the added risk factor is acceptable for training but a fare paying passenger can not be subject to single engine IFR in my opinion.