~]In Islam, gambling is bad so anything related is considered a sin. Also, include promoting a gambling site because they help the owner and attract the attention of other people (no matter his religion) to make sins (play gambling). It doesn't matter if a Muslim doesn't play but if they promote it is still prohibited in his religion.
It's not that I personally care much about it, but I'm sure there are people who do, and I think you should provide quotes and links when making such statements. Some people earn their bread by promoting gambling sites, and some of them are religious. Who are you to call them sinners?
I'm not a part of Islam people but I have found an article saying that gamblers and those who drink wine are sinners.
“They ask you [Muhammad] concerning wine and gambling. Say: ‘In them is a great sin, and some profit, for men; but the sin is greater than the profit.’… Thus does Allah Make clear to you His Signs, in order that you may consider” (Quran 2:219).
“O you who believe! Intoxicants and gambling, dedication of stones, and divination by arrows, are an abomination of Satan's handwork. Eschew such abomination, that you may prosper” (Quran 5:90).
This is not a big deal and no need to argue regarding that matter, we just respect of what Islamic people religious belief and fortunately I'm a Christian and I'm free to gamble either offline or online gambling.