There are differences between the insider trading and your helicopter example. The theory is the better traders know the reality when making decisions, the better. When oil traders hide information about their reserves, they are working on creating a rift between the reality and the public knowledge. Helicopter is overcoming it. When Trump makes empty announcements that change prices in a purely speculative manner, but before doing this he buys futures, he is just creating instability on the market and he exploiting it. Instability is bad, the whole idea of futures is to deal with the risks stemming from the instability.

Instability is bad, but when the cause of it is market getting new information, it becomes ok: it is bad now, but it is good in the long term. But when the instability becomes a source of profits, when there are incentives and means to create the instability, then long term benefits go away.