For the same reason that sending a letter to a friend is different from using the "Send-a-Dime" chain letter. One is something that improves the human condition and brings enjoyment, while the other is a pyramid scheme with negative externalities under the guise of a get-rich-quick scheme.
That is not at all inconsistent with my viewpoint. In the same way that a traditional pyramid scheme enriches a few while the majority lose money, bitcoin speculation can enrich a few before either the bubble pops naturally or it has legislation made against it for being environmentally damaging.
