"periodic and attended use"
How do they substantiate claims that there wasn't someone behind the keyboard at the time of a large download?
If someone is downloading a 2gb file (such as, for example, a massive redux mod for a game like Fallout3 or Oblivion), on 4mbps, it will take approximately 80 minutes (1h20min) to complete. If someone on 384kbps is downloading it, it'll take approximately 15 hours to download.
So now the 384kbps guy, the little guy, is the bad man, because he isn't making periodic, attended use of his connection to download a maximum of 2gb a day as opposed to the 4mbps guy that's able to download in excess of 20gb a day if he really wants to? This, of course, is all while being behind the keyboard (16 hour day), the way many people working from home would be.
What it boils down to is that they need to have some way to get rid of customers they feel are causing them too great a profit loss, but don't want to make any kind of 'abusive use' claims that come back to total bandwidth consumed per month. Because of this they have to find other 'excuses' to cut those people off; ones that, given they are stipulated in a T&C document upon signup by the client, the client cannot argue in court should a legal case be taken out against the ISP 'at fault'.
ISPs that want to have such terms should rather just go with rolling windows to regulate users and be done with it, otherwise there's going to be those out there that will get accounts like Mweb's and use it as much as possible until such a time as they get kicked from the Mweb service, at which point they'll simply move onto the next best ISP to do the same with until they finally get 'banned' from using any ISPs' services except those with reasonable terms that enforce comparatively reasonable, and predictable policies.
Product punting as it may seem, Openweb uncapped accounts would probably fall into such a category, since you may get shapedto**** during business hours, but at least your after hours downloads go along merrily at high speeds, and you can be sure you'll never get a mail from them threatening to cut off your proverbial testicles if you don't relax your usage, and I'm sure there are other ISPs out there that have similar accounts available.