I have not found this to be the case - just last week I signed up for a free 4 month trial of Apple Music and immediately (like, the next 30 seconds) canceled and retained my trial period. I have also done this with News and TV in the past.
Isn't that the free period Apple gives you for purchasing a new device? That's different than the actual free trial in app purchases normal apps get. I think they gave you four months of the service as a specific purchase. Third party apps could do that but then they'd have to manage all of those separate purchases.
That's great for you but I have TV+ and Arcade free trials and both end instantly if I cancel. And I'm not alone, lots of people across the internet experience this too. It's written in Apple's terms.
Replying to myself because I’m past the edit window - the trial I’m referring to was through a third party (Walmart) it sounds as if the terms vary on a case by case basis.