What may be the case is that this is meant to “exhort” Christians to be for the whole Mass …the “should not” being perhaps the language of exhortation.
(This is a first take on the whole thing…so make sure you read the “may” part in the answer)
For taken in the way it seems to be taken in this thread…one should not receive Communion if one has not heard the Word of God. I do not think this is the intent here. But rather to exhort people to see the unity of the Mass and to attend the entire Liturgy (on the norm).
For later …in the Code of Canon Law which came out some 3 years after… notes:
Can. 918 It is highly recommended that the faithful receive holy communion during the eucharistic celebration itself. It is to be administered outside the Mass, however, to those who request it for a just cause, with the liturgical rites being observed.
…it specifically notes that one may for a just reason even receive Holy Communion “outside” of Mass entirely…after highly recommending one receive during the Mass.
And one could ask…from how it seems to sound…it would mean --if taken as a ‘commandment’ that one can not receive if one has not been there for the Liturgy of the Word…which is not the case…for in addition to the above CL later notes:
Can. 917 A person who has already received the Most Holy Eucharist can receive it a second time on the same day only within the eucharistic celebration in which the person participates, without prejudice to the prescript of ⇒ can. 921, §2.
Which implies that one can receive not only outside Mass for the first occasion but also at a Mass where one just happens to say “walk through” at the time of Communion …hence it notes that the second Communion must be at a Mass one partcipates in (that is does not just walk in on…but Mass one is actually attending…)
So it seems to that it can argued that the “should not” here in the 1980 doc is not meant to say “one does not fulfill ones obligation” nor that one “is not to” receive Holy Communion if one misses the readings …but rather as an emphatic exhortation to not willy nilly miss them…but to see them as forming a whole…for Christ speaks and we need to listen! (often times Americans read these things from an “American approach to Law etc”)
So given the fact that theology manuals from the first part of the 20th century will even note that one still fulfills ones Obligation if one misses the readings…so long as one is there from the offertory onwards …and from the fact that the Church has not come out and said one must attend the whole Mass to meet the obligation…or given any “lines”…but rather seems to exhort one to attend the whole Mass and has Pastors exhort he faithful to do so…it seems perhaps reasonable to judge that one still fulfills ones obligation …even if one misses the readings…
(of course remember there can be also venial sin…say missing the first reading without good reason…etc)
BUT be mindful of the fact that the Mass is meant to be a Whole…and Christians are to be at the entire Mass…listening to the Word of God (go read the new doc Verbum Domini from Pope Benedict XVI!)…
But sometimes things prevent this…traffic…kids…etc