KeyPOST report suggests reinforcement is recommended, why?
KeyPOST references Eurocodes 2 ('The Design of Concrete Structures') and Eurocode 7 ('Geothechnical Design') amongst many others, including advice from the Design Manual for Roads and Bridges for foundations in the ground. The Eurocodes are primary Standards used for any structure. As can often be the case the use of Standards can involve an element of interpretation, not least when they are applied to specific situations, and traffic sign structure foundations have different requirements to, for example, foundations for buildings.
Since Eurocode 2 is a primary Standard, intended to cover any structure in the ground, its applicability to traffic signs needs to be considered, and its clauses interpreted in a way that is sensible and practicable for sign structrues. Section 12 relates to light reinforcement that may be appropriate to sign structures, if this was deemed necessary. However, clause 12.1, Note 2 suggests that the section, and therefore reinforcement, does not apply to strip and pad footings for foundations.
KeyPOST is built around the interpretation that a sign foundation is not strictly a strip or pad footing and so Section 12 DOES apply. It would however seem perfectly acceptable to interpret this such as to place a sign foundation into the exempted category of concrete structures, and this is why KeyPOST only recommends reinforcement but does not fail a proposal when this is the case, since reinforcement within the foundations is outside of the scope of KeyPOST.