I think this is because {a:1} is a cleverly disguised block statement with a label named 'a', followed by the expression 1. It looks bad, but it never comes up

I think this is because {a:1} is a cleverly disguised block statement with a label named 'a', followed by the expression 1. It looks bad, but it never comes up in practice because {a:1} is only parsed as a block if it's at the start of a line, which it never would be.