The security argument is not valid on this case,
I can subscribe to online services on my Mac and use them on a browser, be it safari, chrome or others.
Services like Spotify, Netflix, Disney+ and many many others, without exposing my mac to risks, and without having to pay 30% of that money to apple.
The whole amount for the subscription, I pay go to the rightful service provider.
I can use the same service when logged in another kind of computer or device running windows, linux, solaris, android or any other, still not having to pay a cut to Microsoft or Oracle etc.
That includes Apps on iOS on my iPad and iPhone, that are validated ( and hence "safe" ) by the AppStore, and I still didn't had to pay anything to apple because I didn't use In-App Purchases to subscribe to the service.
Why can't we have the same freedom for other service or software providers on iOS ?