There's a reason icon packs are supported by most third-party launchers and a growing number of manufacturer launchers. The problem with those manufacturer launchers supporting icon packs is that they're only supporting their icon packs, but not those sold by Google Play. Samsung's launcher only supports icon packs from the Samsung Themes store. HTC's launcher only supports icon packs from the HTC Themes store.
And that's a problem for a lot of reasons.
Icon packs aren't standardized at the Android level. If you want to learn how to create an icon pack, you won't be looking at Google's Android Developers website, but at guides on forums like XDA Developers. Because icon packs aren't standardized, the same icon pack can look different on different launchers, and some launchers won't recognize icon packs that are built certain ways. In short, the experience of using icon packs can be hit-and-miss.
For third-party launchers, they can list whether or not they support icon packs in their app listing before you download it, and icon packs on Google Play often spell out which launchers they've tested on and support. For the launcher that came with your phone, it's a bit harder to tell users, "Oh hey, your mileage may vary based on the icon pack you use". It's far easier for them to either limit icon packs to ones they've tested themselves, or stop icon packs from being used all together.
If a manufacturer doesn't want to bother with adding icon pack support, I can accept that. What I can't accept is a launcher going to to the trouble of adding and supporting icon packs, but not allowing us to use the icon packs we've already paid for.
The problem with icon packs from Samsung and HTC is that they're often of terrible quality.
I know that Samsung and HTC want us to buy icon packs from them for their launchers. If a pack is truly an original work and works well on that launcher, I'd be willing to buy it from them. Problem is piracy runs rampant in theme stores like HTC's and Samsung's, and most icon packs for these stores don't even theme Google icons — or even all of the manufacturer's apps — beyond a tacky mask. They're pale imitations and don't work nearly as well as their Google Play competitors. By allowing the real icon packs from Google Play to work with manufacturer launchers, you can help cut down on piracy by allowing the genuine articles to work instead of just the fakes.
By allowing third-party icon packs, manufacturer launchers also stand a better chance of users sticking around and actually trying to use their launcher instead of downloading their favorite from Google Play the second they're set up. The on-board launcher on the Galaxy S8 is quite capable for everyday users and themers alike… but the icon packs are awful, and you can't get away from them without jumping to a third-party launcher.
Do we need a platform-level standardization for icon packs? You bet your Android we do!
We've actually seen some manufacturers start to allow Google Play icon packs in recent years. ASUS's Zen UI Launcher accepts Google Play icon packs in addition to the ones found with ZenUI Themes, and BlackBerry Launcher supports Google Play icon packs, letting users give their home screen the look they want without losing out on BlackBerry Launcher's keyboard shortcuts or integration with BlackBerry Hub. While there's plenty of reasons to include support for Google Play icon packs, perhaps the most obvious one for these two launchers supporting it is that in addition to shipping them on phones, both of these launchers are available on (and updates through) Google Play to all Android users. They compete with the rest of the third-party launcher market, and so they have a popular feature other launchers have: icon pack support for Google Play icon packs.
Do we need a platform-level standardization for icon packs? You bet your Android we do! But until we get one, we at least need launchers that support icon packs to support the ones that come from Google Play instead of sitting in their tiny, walled garden of cheap masks and knock offs. It makes manufacturer launchers more competitive. It helps cut down on piracy. It makes users happy. And honestly, isn't that enough of a reason to do it?