Why do I have to pay to watch a show even after I subscribed to Apple TV+?

With the Apple TV app on your smart TV, you can access:

• iTunes Store movies and TV shows (buy or rent);
• Apple TV+ channel featuring Apple Originals [1] [2], by subscription;
• other premium Apple TV app Channels, each by subscription.

Only the content marked as part of Apple TV+ is included with your subscription. The other content will have to be paid for separately.

About Apple TV+ - Apple

Apple Originals - Apple TV+ Press

List of Apple TV+ original programming (episodic/series) - Wikipedia

List of Apple TV+ original films - Wikipedia

There could be a variety of reasons. Sometimes not everything on a streaming service is free (a similar example, I have cable and plenty of channels, but could still access pay per view where I’d have to pay more).

You’d need to contact Apple+ support directly in regards to your specific situation to see what they say.

Get Apple TV+ - Apple Support

Down at the bottom is an option to contact apple support and they may be able to answer your question

Apple TV has free content and movies you need to pay for. You can choose not to watch any that would cost money.

You can discuss that policy with Apple; we are not allowed to discuss their policies.