Best interests means facilitating trades as best as possible, not trying to protect clients from their own trades. No broker is attempting to preserve your capital.
What they will do is protect their capital over yours by raising margin requirements and initiating special rules to limit their risk. They don't care if you blow up your account, as long as you don't owe them at the end.
What they will do is protect their capital over yours by raising margin requirements and initiating special rules to limit their risk. They don't care if you blow up your account, as long as you don't owe them at the end.