Kangkung itself is not poisonous nor toxic. But if eaten raw it poses health hazard because it is a parasite carrier which usually infests human and pig. The problem in US is mainly an invading weed issue I think.
Kangkung is not poisonous. It is illegal in some US states to grow kangkung because this plant is regarded as a weed. A weed is simply any plant that is unwanted. US people don't eat kangkung, so if they find kangkung in their farms, kangkung is seen as a weed. In the same way, US people don't eat or want durian trees, so they will treat durian trees as a weed.
The difference is, compared to durian, kangkung is an easy plant to grow, and it grows quickly (in a month to harvest). The plant is also easy to maintain, so it is easy to see why some people will regard this plant like a weed.
There were also some posts in this thread about toxic vegetables in Malaysia. Before we condemn our hardworking local farmers, we have no other people to blame but ourselves. We demand good looking vegetables, free of leaf holes or defects. Our supermarkets also demand perfect looking vegetables and so do foreign markets. In Japan, the whole cargo of bananas get sent back if the bananas have one black mark/spot too many -- the whole shipping cargo is returned!
Our farmers will be the happiest people in the world if tomorrow we consumers are happy to eat less-than-perfect vegetables. One-third of the farming costs is on buying fertilizers and pesticides, so if farmers can spray less, this means they spend less and earn more. Why would farmers want to spray so much if we prefer some leaf holes in our vegetables?
I have been to Chinese farms in Johor and the farmers there spray pesticides on the day of harvest. Reason? Singaporeans demand perfect looking vegetables.
So, it is important that you wash your vegetables before cooking. Most pesticides stay on the surface, so they can be washed off. The dangerous ones are those that get absorbed into the plant -- which cannot be washed off with or without salt washing/soaking.