For me, the reason is because bitcointalk is home of high rollers. This recent casinos who advertise or running signature campaigns here is trying their luck to got some high roller to play on their platform. They know this forum is where serious players gather, so it's the best place to attract people who like to bet big.

These posts would probably decrease if bitcointalk also bans the signature campaigns of online casinos, similar to what happened with the signature campaigns for bitcoin mixers.