Until 2 months ago, they forced you to have an Apple Watch to be able to watch Fitness+ videos, even if you were already paying for the Apple One bundle that includes Fitness+.
There was no technical reason for this requirement, as Fitness+ is just videos, and even if you had an Apple Watch, you did not need to be wearing it to watch the videos.
While we're on the topic but silly Fitness+ restrictions: Each family member needs to have their OWN iPad to use it with Fitness+.
For example, I have an iPad + Apple Watch, I can use it with Fitness+. My wife however, with her own Apple Watch, cannot use MY iPad for Fitness+. Instead, she needs a whole other iPad set up as her, or it cannot "see" her Apple Watch/has no concept of her.
Family Sharing is under-baked. Fitness+ is also under-baked. The fact they have zero interactive fitness games on Fitness+ is frankly embarrassing, I had fitness equipment 20 years ago which had interactive games tied to your workouts, and Apple with the best programmers in the world, 100x more powerful hardware, and better development toolkits has developed nothing beyond video-recordings of studio workouts.
Actually the fitness-gaming ecosystem is frankly is a terrible state. Currently, Nintendo or Meta are the leaders in spite of them doing little to try.
There was no technical reason for this requirement, as Fitness+ is just videos, and even if you had an Apple Watch, you did not need to be wearing it to watch the videos.