Leave it to DLsite to fuck up DRM implementation.
It's not easy.
Both methods of doing DRM would be difficult
Method 1: Have an API so that developers can access DLSite's database if they can verify an API key.
This is Patreon's method. Patreon creators have access to the API (Application Interface) so that off-site they can still give Patreon rewards by accessing Patreon's database.
This also works for video games as well as most game engines support some form of XMLHTTP which can make basic calls to a website.
If developers really wanted to and took the time, they could make games require patreon log-in in the games themselves as a form of DRM.
The reason they don't is because handling an API is a bit of a pain. Everyone's is different. Paypal's API is different, Patreon's is different. You have to learn the ins and outs of theirs.
You also need quite the skill as a "NETWORK BACKEND" programmer. These are the basic skills college educated folk get that help them land jobs with companies like JPMorgan and Pepsi as programmers. Not many people have these skills.
And if you do this, you're basically making your game exclusive to that platform which is probably not the intended goal. Most finished games are going to go on traditional marketplaces at the end of the day.
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Method 2: Create a general purpose program that has access to the database and encrypt's programs.
Problem is 1, it's general purpose. Not all programs handle encryption the same. Some games constantly encrypt and decrypt their packages during runtime. Encrypting an already encrypted program means it's a matter of priority which program's function runs first, so any time the program needs to access a resource there's a chance something can go wrong.
And the second problem would be that programs that run in that manner usually add a lot of overhead to the internally running game. This kind of DRM would absolutely destroy games like Skyrim where most of the CTD(Crash to Desktop) issues happen in the loading screen because of asynchronous functions (trying to load a resource that needs another resource to be loaded first, before the first resource is loaded)
I'd prefer method 1 personally, but Method 2 makes it easier for the end user, and DLsite can actually sell this ease of use as a feature to developers concerned about piracy.
Steam is technically method 2 as steam has a special loader, but it's very lightweight. It changes game files very little.