This is fro m the BBC iplayer website
BBC iPlayer Help
Why have a time limit when I can record a programme with my PVR?Some users have asked why the BBC has limited the period of time you have to playback a programme when there are no such restrictions if you use other services/methods such as Sky+, a Freeview PVR, or personal recording on a DVD recorder or VHS.
These other services or methods rely on a 'time shift' exemption that permits you to record broadcasts without infringing the Copyright, Designs and Patents Act 1988, as amended by the Copyright & Related Rights Regulations 2003. This exemption permits personal copies to be recorded �in domestic premises� and solely for the purpose of enabling the recording to be viewed or listened to at a more convenient time.
The difference with BBC iPlayer is that the BBC makes available its programmes to users as an On-Demand facility, that can be accessed from anywhere within the UK (domestic or otherwise) and so in other words, you cannot rely on the time shift exemption when you use BBC iPlayer.
This means that to offer BBC programmes through BBC iPlayer, the BBC has to acquire the relevant rights from the people who make and appear in them. Like other broadcasters, the BBC does not always own all the rights to the programmes it broadcasts and must use technology to prevent programmes being copied and distributed illegally.
In the vast majority of cases, the BBC and rights holders have agreed a limited period where UK users can watch programmes either as an On-Demand stream, or temporary download using DRM. It's rather like borrowing books or DVDs from your local library for a set period of time.