The supplier of some restricted modules won't sell you again these modules only because you defect and became an enemy? Seems logical to me.
They don't sell it to you. They sell it to the insurance company.
Well, if your ship is blown up, your modules are gone. Yes, you can claim your insurance, but the only supplier of these faction modules is the faction itself. Why should they still sell it to you after you defected? They would tell you to shove your insurance voucher up some black hole.
Let's look at it from a different angle:
You spend a month trading/fighting or doing whatever grind is close to your heart in order to buy a better ship.
Then you immediately splat it against a T9 while boosting out of the mail slot.
Well, if you were forced to spend another month just grinding to recover that ship, eventually you would figure out you've been spending too much time doing busywork instead of having fun, and you'd probably lose interest in the game.
Since games are supposed to be fun, FD came up with a magical solution that works out splendidly for almost every one involved: an insurance option that will basically give your ship back to you for 1/20 of its current market price, in a single payment you'll only have to make when you need it.
Please notice that it doesn't benefit anyone other than you - and it doesn't make any sense whatsoever.
There you have it: a whole galactic corporation that magically operates at a 1,900% loss in every single transaction they make, just to keep you happy.
You accept that and make ample use of it - even though it is a completely absurd proposition.
Yet, somehow you expect them to follow very strict rules when it comes to PP modules.
Some people would call that "cognitive dissonance". I'd call it something else entirely, but then I've got a warning already.
This would achieve that players stick with their faction and don't just pick the flavour of the month to get the highest profit.
Ah, being THAT innocent again.
Nah, a module that's mostly a side grade in a different color isn't enough to keep people spending month after month hauling fortification supplies at a huge credit loss, while suffering constant interdictions and accruing bounties on their heads from undermining.
Take away the ability to collect modules and you'll make PP even more unappealing to the average player.
What this would achieve is that a lot of players would just abandon PP altogether, since most people aren't really interested in playing politics.
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If the unique shield module is all you care about then I guess yes, Aisling is your faction.
Even though he's doing only the bare minimum to get the prize, while doing everything he can to hurt that power in the process?
Because that's the point he's making: it doesn't matter if you're an asset or a drag to your power, the rewards are exactly the same, which suggests that in its current implementation, PP is deeply flawed - and not really a lot of fun.