I often start a movie, and for one reason or another, can't finish it until a couple days later. I know of no other rental service that limits you to once starting a movie, requiring you to finish it in a 24 hour period. Lame.
It is what it is, and is almost certainly driven by the US movie studios, since Apple gives 48 hours in other countries. Either live with it or use some other rental service. Amazon at one time had exactly the same 24-hour viewing period, but I don't know if that still applies.