When you have a Future position, and you place a target order and a SL order, both these orders are considered to be orders placed against the position you are holding and hence does not ask you for extra margins.
What happens in Options is, if you hold a position, you can only place one order - either a SL or a target order. This is because once you have placed one of the orders, the other order is considered as an order to short options [one which allows you to create a new short option position] thereby blocking additional margins.
Excerpt from our post on Z-Connect "This logic [2 orders against one open position] will work for equity, equity futures, commodity futures and currency futures. This would not work when trading options. The reason it will not work when trading options is because unlike in equity trading and futures trading, the margin required for buying and writing (selling) options varies. If you have Rs 5000 in your account and you bought 1 lot of Nifty calls at Rs 100 (Rs100 x 50= Rs 5000), you can place the first exit order, but for placing the second exit order it will ask you for the option writing (selling) margin." (http://zerodha.com/z-connect/tradezerodha/margin-requirements/margins-2-exit-orders-for-1-position)