Because Zorro developed it on his own, he has costs involved, he takes time to give support, he wants to be able to develop it further, all this costs money, and it is the only way for him to get back the money spent (although he probably wont ever make up all the time/effort that was put in + research as he knows the AT/GSM spec better than anyone I have seen!) and it will give him money to be able to add more features, and possibly integrate better if he can ever afford the BT SDK.
Basically it is the same reason that microsoft charges, but of course on a MUCH smaller scale. Lets not start discussing this here. He gives everyone the option of using the older version for free, so I dont think there should be much discussion.
I hope this clears up misunderstandings, and I hope I'm not too far off, am I,Z?