Re: Use of the word "should" in OWL Rec

In addition to the possibly inappropriate use of the word SHOULD in 
Section 3.1.2.1.1 owl:allValuesFrom (cf. the original posting), I hit 
another passage in Section 4.1.1 rdfs:subPropertyOf 
(http://www.w3.org/TR/owl-ref/#subPropertyOf-def):

"[..] Formally this means that if P1 is a subproperty of P2, then the 
property extension of P1 (a set of pairs) SHOULD be a subset of the 
property extension of P2 (also a set of pairs)."

Comparing this with the semantics in 
http://www.w3.org/TR/owl-absyn/direct.html#3.3, which states that ER(p1) 
⊆ ER(p2), the use of SHOULD here again seems misleading.

Could someone be so kind and comment on why SHOULD is appropriate in 
those sections?

Thanks!
Martin