Are stock 'bandars' legal?
Having large capital and trading actively is legal. What's illegal is market manipulation like pump-and-dump, spoofing, or spreading false information. OJK and IDX actively monitor and penalize these violations. So a 'bandar' as a big player is legal, but a manipulative 'bandar' is a criminal.
How do I protect myself from market manipulation?
First, focus on liquid stocks with large market caps — harder to manipulate. Second, always have a cut-loss plan. Third, don't be tempted by stocks rising 20-30% in a day without clear reason. Most importantly, don't invest money you can't afford to lose in speculative stocks.