A premined coin isn't a scam, but when the premine dumps on the market and the coin is abandoned the same day, then it's a scam and experience has shown that the premine dumps more often than not. Sometimes the premine is traded out in a civilised manner or dumps months after release which doesn't make it better. A sensible investor would stay away from premine because it only ads risk and there is options for the devs available to pay themselves over time like in the code of devcoin or freicoin so a premine can actually not be justified. Exactly because these other options exist a premine is a red flag. Honest devs have other options to raise funds.