Based on the shape and the pricing per square metre, it looks like the primary use case is for pavement.

As for being a "horrible idea", this is what happens to plastic today in the slums of Nairobi:

https://media.gettyimages.com/photos/kibera-slum-nairobi-pic...

Even if the bricks leach a bit, it's still much better than the status quo of soaking individual plastic bags in sewage for years.