Why do "const inout" and "const inout shared" exist?

[ag0aep6g via Digitalmars-d]

On 07/01/2017 11:47 PM, Andrei Alexandrescu wrote:
> Walter looked at http://erdani.com/conversions.svg and said actually 
> "const inout" and "const inout shared" should not exist as distinct 
> qualifier groups, leading to the simplified qualifier hierarcy in 
> http://erdani.com/conversions-simplified.svg.

This may be a stupid question, but those graphs say: inout -> const, but 
inout may stand for shared and there's no shared -> const.

How can inout -> const be allowed while shared -> const is forbidden?