It was not a burst test, it was an overspeed. The underlying premise is that the engine only bursts when oversped (an assumption not borne out by history but accepted by the French). The EC 225 airframe is not capable of protecting its vital systems from a true burst.
As far as I can find out, ALL engines must be certified to stay together and not burst, at least those currently flying (FAR 33.75 and .77). In FAR, the ability of the Aircraft to withstand the burst is not tied to the ability of the engine to not burst. To explain that the engine demonstraton satisfies the airframe requirement is another example of grandfathering the latest regs, I think.