From previous manifestations of the surcharge I remember a couple of things.I also don't understand why this has to be implemented as a "surcharge" instead of increasing the plan amount.
1. by adding a surcharge, one per account, people who have multiple services with Exetel only get one increase in payment. If they increased Plans then it is likely that you will get a bigger increase. (Multiple increased plans)
2. by keeping their plans and fees to an absolute minimum all the time, Exetel is more susceptible to financial problems if something goes wrong with the deals they have with their wholesalers and therefore by adding a surcharge, usually for a temporary period of time, alows them to ride through the rough period. We are still paying less overall than we would if we were with another ISP.
3. by adding a surcharge rather than an increase in the Plans, then it is easier to remove the surcharge later when the 'financial crisis' has passed.
It is never nice to sign up with a contract only to find that the deal has changed within weeks of signing up. However, my experience is that the problem is usually with Exetel's inability to explain the situation to us in a 'nice way' rather than there being any sneaky underhand manouvering happening.