The problem is that the fair usage agreement isn't really an agreement. To agree on something you have to know what you're agreeing to. If MWEB won't divulge exactly what is abusive ahead of time I don't think they are entering the deal in good faith and are not making it reasonable for anyone to know whether the product is for them or not. Cut offs should be fixed and mentioned firmly in the TOS. Imagine buying insurance cover marketed as unlimited but finding out that the company will only pay for 25% of your stolen goods. That would never fly.
Did you actually read MWEB's AUP?
http://www.mweb.co.za/legalpolicies/GeneralPage/AcceptableUsePolicy.aspx
Some quotes for you
"MWEB reserves the right to establish policies, rules and limitations, from time to time, concerning the use of the IP Service. You must comply with any bandwidth, data storage and other limitations we may impose, in our reasonable discretion. Failure to comply with these rules will result in your service being restricted, suspended or terminated, in our reasonable discretion."
"You may not restrict, inhibit or interfere with the ability of any person to access, use or enjoy the Internet or the any services, or create an unusually large burden on our network, including, without limitation, continuously uploading or downloading streaming video or audio; continuous FTP uploading or downloading, or otherwise generating levels of traffic sufficient to impede others' ability to send or retrieve information, or to use the services in an abusive manner in connection with any unlimited packages, options or promotions."
"We reserve the right to establish policies, rules and limitations, from time to time, concerning the use of any service. You must comply with any bandwidth, data storage and other limitations we may impose, in our reasonable discretion. Failure to comply with these rules will result in your service being restricted, suspended or terminated, in our reasonable discretion."