Most brokers have their own broking firm, still they charge brokerage too *wink*
On a serious note, having our own DP does not make the costs related to DP services go away. Some brokers might not be charging any AMC but their brokerage and demat transaction charges are higher. The idea behind discount broking is that you pay only for what you use and pay less for each service. For traders, savings in brokerage would be much more than the cost of AMC (come to about Rs 25 per month).
Also, the AMC is not 300+. It becomes 300+ due to service tax.
We all wish the taxes could be lower but doesn't always happen.