BT don't have a choice im afraid, its competition laws. If the line is present, but not connected, then its not under anyones control as such.
Don't confuse BT Wholesale, who own the lines and network infastructure and Openreach, who maintain and connect the lines, with BT Retail (ie. the company you always see advertising their broadband and telephony services etc) - though they are all part of the BT Group, they are seperate entities and BT Retail are simply a provider just like Madasafish or Tiscali etc...
Unless BT Retail have the line in use, and a contracted end-customer, BT have no way of preventing you from going with whoever you want. If you get the line connected with Madasafish instead of BT Retail, it is still an Openreach engineer who does the reconnection for you, and they still lease the line off BT Wholesale to sell on to you, it's simply that they charge £49 for the engineer visit (if required) rather than the £115+vat that BT Retail charge for the same service.