Page number? Sure. Let's go with 52.
"An armor or weapon proficiency you gain from a class counts as the appropriate Armor Proficiency or Weapon Proficiency feat (see Chapter 6)."
It counts as the feat and then directs you to go read up on Chapter 6 (Feats) for more info.
I'm not saying I like it, and I'm not saying I wouldn't change it, but it is exactly how it is written.
It isn't a bad interpretation. The armor proficiencies count as those feats and if you lose a prerequisite for a feat it turns off. They worded it perfectly to punish people.
"Well, in the last edition..." isn't a meaningful argument.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.