I understand why you would think that. 

But going over 1 is only "wrong" when a value of 1 means that the surface reflects 100% of any light falling on it.

In this shader this value has no such meaning.

There is lots of other maths in this shader, and the value I am adjusting is just one of several factors. Setting it to more than 1 does not introduce incorrect physics, and more to the point it looks much more realistic.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)