Question about rejects by serving agents

Hi,

In 3.7, a serving agent "MUST reject any article that does not include
all the mandatory header fields or any article which contains header
fields that do not have valid contents".

Is there a reason why a serving agent MUST reject such articles?
I understand that injecting agents MUST (because it happens at
the beginning) but what surprises me is that it is a SHOULD for
relaying agents.

Wouldn't it be better to have either MUST or SHOULD for both relaying
and serving agents?  No need to relay the article if it MUST be
rejected at the end, isn't it?

-- 
Julien ÉLIE

« En voyant le lit vide, il le devint. » (Ponson du Terrail)