Siddharth Patel, Nice Question!
As per my understanding this makes sense because via Rest API; we are actually checking the Service definition. In Thingworx composer when a Thing is created based on the Thing Template; we can run those services and GetImplementingThings Service doesn't apply to a Thing.
But this service is available while creating a new service and Mashups for use.
I hope it helps.
Thanks for reply.
I get your point that had those services been visible as part of Template, then they would get inherited by Thing , but Things should not be allowed to run GetImplementedThings service.
I agree with your point, though I don't agree with the approach. Just as those services can be made visible in mashup, it can be made available only in ThingTemplate (probably with the star mark of some kind - denoting that these properties won't get inherited ).