If you look at what NAWAPA was trying to do, it was very different. They wanted to dam SEVERAL rivers and divert them. Many of the complaints were about disrupting the flow of the rivers.
NAWAPA wanted to divert rivers. The Bengal plan does not divert rivers.

The Bengal plan dams the whole bay. The rivers are completely untouched. The equivalent of this would be damming the Arctic/North Alaskan areas where the rivers drain, and then sending that water back into North America.
(no such proposal was ever made, probably because these drainage areas are SO FAR away from the Arizona desert--Asia lucks out here because Bengal is so much closer).

No, it's NOT the same logic, because when you divert a river, downstream flow is affected and the water's altitude is lower. Certain branches of the river can go completely dry because of this.

NAWAPA was purely about water. Bengal is about water, but ALSO about preventing the catastrophic flooding that NECESSARILY WILL happen to 300,000,000+ people if you do nothing. Building it is a twofer, basically

Also, the Bay of Bengal gets 2.5x more rainfall than the Alaskan coast. And the Brahmaputra-Ganges-Meghna's discharge is 4x bigger than all the combined rivers of Alaska/Canada. So the bang-for-the-buck is way higher too.

And in addition to all that, China and India simply need more land, far more urgently than the US. China also has far more capacity to build something like that than the US.

The Bengal plan WOULD have environmental effects, but only for the actual Bay reservoir area. Not for the land/riverine areas. And the effects would simply be a matter of quality, not quantity. Rather than forests running dry and being deprived of a precious resource, it would simply be a seawater habitat turning into a freshwater one. The engineering would be equally unimaginable though, yes. But the EU is looking at a North Sea dam which would actually be a lot more difficult to build than this.