Because it announces that you intend to use the name 'A' in the current scope after some assignment happens in a nested one. It's intentional.

It /can/ be useful, but it should be opt-in. JavaScript had to figure this out with their 'var' declaration scoping too.

Also, try:

A = (predicate) ? 3 : 5

Or, in a language that embraces expressions instead of statements:

val A = if (predicate) { 3 } else { 5 }

These are all more intentional, concise, and readable than allowing variables to outlive their scope by default.