Yeah, thats not strictly true as these aren't full codexes so you need both Codex Marines and the Chapter Codex.
It's bang on true. 3rd ed had exactly the same set up. With a main vanilla marine dex and then supplementary dex for each specialist chapter.
I mentioned 2nd ed because each chapter needed it's own specialist book apart from Angels of death which had two chapters in it. Not the same, I agree.
So it has been done before is all I was pointing out. Which is true.
He got me unprepared 
BTW due to some unknown glitch in new and otherwise very cool Necromunda expansion (there are rules for use robot from Blackstone Fortes as a black market Brute for example)
- The Enforcers (Neo-abitrators) cannot use nor buy their most trademark item - manacles 
I wouldn't say it is their most trademark item. I would suggest shock batons would be those. Or the combat shotguns and bolters of old. It is also not entirely true.
There was even Community article about top 5 Enforcer equipment listing Manacles as Top 2 IIRC 
But somehow they are only gang that cannot buy them in the rules and every other gang can buy them on black market 
Necromunda groups are puzzled and I find it very hilarious

As with a lot of things these days, folks like to jump to conclusions before the read through stuff properly and flake out over it.
They don't have manacles but they have access to a drill that replaces the Coup de Grace action with one that removes a model from play in close combat like they had succumbed to a webber attack. That is to say they have been hogtied and prepped for collection to be taken off by the law. So essentially, they have been manacled. For the purposes of the rules it does exactly the same thing as models that have been webbed in the post game sequence.
This is exactly what the manacle rules do. So I suspect what they are saying here is that they either follow a code of killing on sight or they try to arrest. Which fluff wise makes more sense to me.
Either your police force is going to shoot first and worry about prisoners when they find someone still skulking around after, or they will seek to cuff and detain folks at the first opportunity to do so.