The nature of ADSL is that it is a network just like any other.  Many people have more than one computer at home connected together via a Local Area Network, you can transfer data between your home computers easily and quickly.  When you sign up for an ADSL service your provider applies some changes at the local exchange which make your "house" part of their Local Area Network (I use this terminology loosly).  What this means is that your broadband modem, containing your username and password, can not be used at a friends house if they are with another provider because you would no longer be apart of your providers network.  The line into the house is physically bound to someone else.

Ok, are we clear on that?  When you get data transferred to your computer from any other computer on that same providers network you are not getting it from the Internet - it is only coming in from the Internet if the data is coming from another provider.  The Internet is nothing more than a lot of smaller networks joined together via routers, cables and other cool stuff.

Example 1:

Person A wants to send a 250 Megabyte movie to their friend, Person B, and receive a 250 Megabyte moive back.  Person A is with provider XYZ and Person B is with provider ABC. The two movies are transferred via the Internet because it is leaving 1 providers network for another.  If both people had a 200 MB Download Limit then they are both 50 Megabytes over there limit.

Example 2:

Same as above, except both Person A and B are with the same provider.  The data is never on the Internet because it stays within your providers network.  Both have received their movies but the provider is being paid twice for the excess usage.

In my opinion example 2 is double dipping and being paid twice for the same data.  I am going to do some more digging and see what I come up with.  I will be reading providers contracts and examining the words used to describe data transfers.

Once upon a time we were only charged for downloads but providers have discovered that they can make more money by charging for both down and upward data transfers.  Crooks! 

In simple terms - if the same scheme was used at Australia Post, you would have to buy a 50 cent stamp to send a letter and the person receiving the letter would have to buy another 50 cent stamp to receive it.