The SEBI circular on this came sometime last July, it very clearly said that if equity segment has to be enabled on a new trading account, there has to be a demat account mapped to it. My guess is that your account was opened before that.
Why does the demat have to be mapped?
What if you have bought stocks for intraday and it hits downward circuit? The broker has to take delivery of the stock and if he does where does he send those stocks to if there is no demat account mapped to the trading account?
If he keeps it in the pool account, it is a violation of rules.
Hopefully clarifies...