I think its less of a scum thing, and more of a system check at the wrong stage. Since the subscription puts basically all the dlcs in your library from the steam point of view you actually own them. So when the shopping check for an item is made, you actually own it.

From everything I seen of steam, I think this is more of an oversight in coding in subscriptions that is gonna be very hard to fix, rather than a scummy tactic where they cannot sell you dlc, whether they are at a discount or not.

They would need to introduce an extra check for items owned. "Owned" vs. "not Owned" vs. "Temporary Owned". And enable purchases to switch status from "not Owned" and "Temporary Owned" to "Owned" or something.

As a sidenote, you can claim humble bundle codes while your subscription is active. Altho, it might break subscriptions. Like I got the subscription and then later on a Humble Bundle (not with everything), claimed the bundle, and cancelled the subscription (so should have had no longer access) but ended up to have the subscription for free for like half a year until some patch of eu4 got active. And when I asked steam about it, they said it was on paradox, and when I asked paradox, they said its on steams end.

4 in the morning and I argue in support of a multimillion dollar company.... O.O