Hahahaha! Man, what a tosh, unsafe condition?! so you also never give a student a failure and feather and secure exercise?! And then airlines complain about declining training standards
This procedure has been standard at the BAS, Belgian aviation school, later Sabena Flight academy. You call them dangerous?
Idling an engine on a twin is not at all representative to an actual failure. the full procedure must be trained onboard the actual aircraft.
Not unsafe at all, and not bad for the engine
It looks like the operators you mention have not bothered to learn from the experiences of others. The following extracts from the US NTSB (Google it if you don't know what it means) are applicable to your statement.
National Transportation Safety Board Warning On Simulated Engine-Out Maneuvers
The fatal crash of a light twin in which a flight instructor and an applicant for a multiengine rating were killed, prompted the NTSB to issue an urgent warning to all pilots simulating an engine-out condition on multiengine airplanes.
The Board's investigation revealed that some flight instructors do use the fuel selector or the mixture control to shut down an engine to test the applicant. Although this is a recommended procedure, the urgent warning was aimed at flight instructors who were using this proceure at altitudes too low for continued safe flight. The NTSB observed that use of such procedures at traffic pattern altitudes may not permit instructors enough time to overcome possible errors on the part of the applicant.
The recommendation by the NTSB means that all simulated engine-out operation at the lower altitudes should be accomplished by retarding the throttle and this should be done slowly and carefully to avoid engine damage or failure.
The Lycoming Service Bulletin No 245 stated that if the power was abruptly terminated, it must be accomplished with the mixture control. Of course, this was intended for the higher altitudes where a complete engine shut-down could be conducted safely. However, any practice of simulated engine-out condition at low altitudes should be best accomplished by a slow retardation of the throttle in accordance with the NTSB recommendation. This careful techique will protect the engine, and at the same time provide for instant power it it is needed.
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Instructors would indeed be foolish to disregard the advice given by the NTSB. Legal action against the flying school and instructor would likely follow if this recommendation was ignored causing an accident