I imagine energy weapon armour to be a bit different than physical contact armour, and while it would be possible to incorporate protectiveness for both, it's unpractical - hence the lasgun armour penetration, armour is generally unprepared for them.
Fluff is fungible, and it changes over time. Still, from the Confrontation rules: "...Lasers are superficially damaging rather than penetrating weapons, although a laser hit to exposed flesh is often more dangerous than a hit from a projectile weapon. The most notable feature of the laser is its inability to pierce thick armor."
For lasguns on Ratskins from fluff perspective - I wouldn't say lasguns are the AK-47 of 40k. Not sure what to give as a counterexample, what I'm thinking is, ~everybody has one, ~everybody can use or quickly learn to use it, barely anybody knows how it works and/or can make their own. Firearms can be made sense of by disassembling them, maybe proxying some parts - that's the AK-47, and I think some Autoguns in 40k. I imagine disassembling a lasgun would uncover some high precision optics, electric and electronic parts. Intimidating. If Ratskins don't trust the hivers and their contraptions - lack of lasguns can be justified by fluff.?
I've got to disagree on this one. From the rulebook:
"Laser weapons are manufactured in vast quantities in the hive city and are exported to the armed forces of the Imperium throughout the galaxy. Lasguns are traded in Underhive settlements and the Guilders bring shipments from the factories above. Power packs come from the same source and some are made locally. Supplies of parts are plentiful and repairs easily effected. Laser technology is reliable and easy to replicate, and although the weapons are not the most powerful they are certainly the most trustworthy."
This sounds exactly like an AK-47 to me. Outlanders makes it very clear Ratskins are perfectly willing to trade with night caravans and unscrupulous Guilders, and Ratskins can't have too much trouble with using stuff from Uphive such as Meltabombs and all the other goodies from the rare trade chart. Presumably the Guilders and night caravans have loads of lasguns for sale. As for your concerns about complexity, I'm not sure that a simple lasgun is any more complicated than a rifle in the 41st millenium. It's been a while, but I think the Imperial Infantryman's Uplifting Primer is clear on this point...lasguns have to be easy to use, disassemble, and maintain, or they wouldn't be the weapon of the Guard. I don't think you're right in your statement that barely anyone can figure out how to make a lasgun. This isn't an Ordinatus of Mars. Lasguns can't be hard to make, as according to the fluff, Necromunda churns them out for export in vast quantities.
As I said, fluff changes, and we can find whatever we need to support all kinds of different, even contradictory statements. Gameplay has to come first. If there is some legitimate reason why Ratskins can't take lasguns, that's fine. Personally, I don't see one. If there is some strong reason to give lasguns -1 to armor, that's fine too. I always thought of that as a leftover from the conversion from 40K...a game where the -1 was necessary for the Guard to have any chance to win against Space Marines at the time. Lasguns are plenty good without the save modifier, and armor, at least in my small circle, is taken rarely enough due to loads of weapons totally negating the saves that this small adjustment seems like a good idea.
