That is just an artificial restriction. There's nothing stopping on demand content from working with a fast forward button, other than silly licensing agreement nonsense.
Traditional cable providers charge you for DVR because the cable guy comes and brings you a piece of equipment, with a hard drive inside, which you agree to borrow from them to record your shows on. There's no DVR charge if you buy your own TiVo DVR, aside from the CableCard required for it to work on their system.
Except for that $13 a month TiVo charges you!