I believe this is because on Youview the core catch-up players (BBC, ITV, Ch4, Ch5) are provided for free. Even if you cancel your BT TV subscription in future those catch-up players will still work. They also work on retail Youview boxes where the customer is not paying any subscription. I assume they are funded by the advertising.
On the older BT Vision platform the catch-up players are provided as part of the subscription. If you stop the subscription they stop working. This means the provider can remove adverts if they wish to, and BT do remove them on the BT Vision platform. Note though that the selection of catch-up programmes on BT Vision is I believe more restricted than that on Youview. So Youview customers win on that front. Not sure about on Sky.
So far (with 4OD) they helpfully give you a countdown to when the programme will start again, so you know if there is time to make a brew or just put the cat out....!