said by tshirt:But it's not really extra ...
Probably a bad choice of words?
Order only telephone service from a LEC, or only television service from a MSO, and they cover line maintenance out of the bill. Add HSI, and there is no need for another line maintenance fee to the customer with bundled services. So the HSI service is priced without consideration of line maintenance because the customer is already paying for it.
Since the HSI price does not cover maintenance costs, unbundling HSI incurs an unrecovered maintenance cost for the service. Unless the LEC/MSO add a fee to cover maintenance costs.