Because in the current market "unlimited" refers to download caps, and there is not one of these on the service.
Although i had reservations about the idea in the first instance, i don't see a problem with shaping between the hours stated. As internet usage patterns change it will be more important for people to get better latency (gaming, streaming, VOIP).
If people want to leech and leech that is fine, but just if they try to do it between 6pm and midnight then it will just run slower
When i used to p2p at uni then i normally left it running overnight anyway, by normal (not student) sleeping hours that will leave 8 hours of unshaped leeching time...