This very clearly says that SHOULD is not effectively REQUIRED at all, and is fact nothing more than RECOMMENDED. Really not sure how you misinterpreted this so badly
It's not required but they need to understand the implications. In this case the implication is that Google drops the mail. So clearly they didn't understand the implications.
Google probably did parse these messages as well-formed before inspecting them and deciding to drop them based on the lack of this field. The RFC imposes no mandatory obligation to deliver messages just because they are well-formed.
