Wikipedia has a good defination of
Traffic Shaping.
So if it is PURELY on the grounds to let time sensitive traffic such as VOIP and Video on demand through then yes of course its justified. However most ISPs who use it and port throttling to control usage of things like BitTorrent certainly arent justified.
If you pay for "Unlimited" you should get unlimited in what ever way you feel. Saying that I have no problem if Bit Torrent is shaped and throttled IF the ISP is upfront in saying so, and only does it on a timed basis.
My ISP I get 2 caps, a peak time cap (8am-10pm M-F) of 60GB a month, so I dont Torrent or at least throttle it myself, giving the average every day user the bandwidth to do what they like without the network being hammered, however my off-peak cap (10pm-8am M-F and all weekend) is 330GB, and I know I can do what I like.
All of this is explained up front, I know what the rules are, and their usage reports help you keep a track of things.
Jen