.that's a really good insight and a possible reason for their actions, the reason why I say this is because attention and focus is needed whenever it involves group chat and most especially when it has to do with gamblers. Aside from scammers, inpatient gamblers can cause lots of damage to the casino reputation especially when they ain't that active in terms of response and attending to issues concerning the casino.
More than just this, I think they are trying to test things out with their telegram.
I am already on their telegram group and I see some users their mentioning that they are from bitcointalk forum. You known when you exploring some options reaching and retaining new customers you want to make it as calculated as you can be to prevent you from losing it to scammers who are already like a virus everywhere.
But somehow its not good to close the community exclusive only for small groups of people. Since how they can grow their community with that? Telegram scammers are just there and I think its personal obligation for people to know what is scam and which is not.
Its better if they just warn people about those scams and also always highlight in the group that they won't send any message first. Also I don't think they should get worried about telegram scammers since what's really better is on how they can expand their community. But for sure they could find good solution with this and provably will open their telegram to public so that people could join directly without waiting for invitation from them.