These are two completely different situations. For one thing, it is much easier to prove that someone is spreading false information about Bitcoin than to prove that a casino is really scamming its players. Don't get me wrong, I believe 1xbit is an obvious example of a scam casino, but it's hard to prove without a doubt without information they won't share. Secondly, this is the BitcoinTalk.org forum, so the main emphasis is on Bitcoin discussions and spreading information about Bitcoin and related technologies. Besides, advertising is only a secondary feature of the forum, and each member can choose to ignore certain members or disable signature advertising altogether.
That is because bitcoin is legit and you & me is a bitcoin user so we know the truth but for a rumored scam casino we are not sure if what to believe unless if we tried it and we have been scammed. That is the only way to agree that it is really a scam casino but will you wait before that happens and before many people lost their money only to prove it?
The negative tags, screenshots/proofs, and left & right warnings are already enough I guess to agree that the alleged casino is really a scam. In terms of ads, it can be a main thing too because this is where the forum gets money to continue its operations but there is a warning about ads that they are not endorsed by the forum so some of them may not be legit.
Also you can disable both the forum ads and user signatures if you want to. The forum gives you that option. Scamming has been a very gray area in this forum anyway. If you commit plagiarism, you'll get banned but if you scam someone you'll only get bad reputation. This never changed. A scam casino come here and offer their services. They will only get negative trust ratings and some flags and they'll continue to operate freely...