I forgot to add a few additional things.
There are a hierarchy of documents loosely called the Tech Regs. The top level are in WMO 49, Vol 1 to 4 and annexes. These are the standards and recommended practises. The publications below this are called Manuals and they also include regulatory material, they expand upon the material in WMO 49 but do not introduce any new standards or recommended practises. Below the Manuals are the Guides which are designed to assist Members implement the Standards and Practises and definitely do not introduce new regulatory material. The words Shall and Should have prescribed meanings in WMO 49 and the Manuals but normal usage elsewhere.
Now for a body like WMO (and its predecessor IMO) where surface observations have been its bread and butter for nearly 150 years you may expect instrument characteristics / performances etc to be in the tech regs and be Shall. Interestingly enough in an age where we are very concerned about surface temp as a litmus for climate change the CIMO publication setting out instrument specifications is a Guide and there are at least 3 different types of shelters for surface temperature instruments. It has been this way for more than 60 years. This still staggers me. So I can strongly relate to your frustration and to me our issues are very small in comparison to this much more fundamental question.