Ukraine, Russia and the US are not signatories on the ban. It doesn't take the ethics out of the equation, but each country has taken a stance on the subject. Of course Ukraine is already on record as using them on its own population.
In regards to the $900 billion, it's not about how much money, it's about capacity. I've previously provided supply chain information from our government to you actually, and America simply doesn't have the capacity to keep supplying Ukraine with the weaponry they have been. The ramp up to that capability is going to take years.
I don't know if it's the cause, but I speculated that's what the early "explosions" in the Russian factories were about. The ramping up of production from one shift (8 hrs) to 24 hours.