mm
Only Islam of all religions (that i am aware of) prohibits gambling directly. And i wouldn't use word "Haram" to describe what other religions prohibit as that word is connected to Islamic law and that wouldn't apply in anywhere else.
But as an agnostic, i see it as only positive when people aren't that strict with their own religion's rules. Also, you wouldn't know if they are betting when they are just promoting the site.
Some Christian preachers also preach against gambling even when it is not explicitly written in the Bible. I am aware that only Islam condemns gambling in its Holy Book.
You are right, Muslims should not gamble. If it is a sin in your religion, then abstain from it. Unless you are not a faithful Muslim.