The reason for this is fairness in the marketplace. BT were given an unfair advantage at the outset. BT still retain this advantage in that many other telco's have to deal with BT and relatively substandard service.
We are now faced with a requirement to massively upgrade the telecoms infrastructure, BT has dipped into it's deep pockets but wants government money (hence the 50p tax per line) to provide broadband services to the populous. This infrastructure is likely to be paid for by the tax payer and then charged out to the user to swell BT's coffers.
It is time that telecoms infrastructure was a totally separate company to that of any current telecoms provider this will provide better competition and better service for business and domestic consumers alike.