UOM remains locked when using write-in products

Something I have never noticed before but when using write in products its not possible to select a Unit of measure. I wasn't sure if this was a customisation in our environment or not but the functionality is the same in a vanilla install. 

I cannot make the field editable via configuration, but have yet to try using JavaScript or business rules. Prior to doing this I wanted to understand the reasoning behind the field being locked when using write in products. To me it does seem like a requirement that should be supported.

There doesn't appear to be any mention of this elsewhere on the net?