It's not a premine in the literal sense of the word, where the developer keeps all the premined coins for a profit. Stack coin is a 5% IPCO (Initial Public Coin Offering) which means the first 5% of coins to be mined are sold to investors (which anyone with at least .1 BTC can be) and none are kept by the developers. In many cases, this can actually lead to a greater distribution of coins than a non-premined coin.

Say for instance a coin is announced with a large gap of time before release, this allows miners with large amounts of hashing power to take advantage of the early mining difficulty, securing them a large percentage of the initial mining. With Stackcoin, miners still have that ability, but the first 5% will already be divided up between many investors (currently ~40). Another advantage of an IPCO is that the developers are able to generate money before launch without any negative backlash or any fears of dumping by the developers. Also, with great developers like the ones Stackcoin has, the proceeds of the IPCO will go directly into supporting the coin and furthering its market share.