Crocodile tried to take over Alabasta from the shadows and with secret agents, by tricking the people into a civil war and taking over the kingdom in one fast, brutal move.
Moriah was hiding in the fog and tried to build his Oz weapon in secret. For most people his action were just Ghost Stories.
And both had the title as Shichibukai to fall back to, if they encounter the Marines. Being a Shichibukai in the Old World makes a lot sense. Your protected from the Yonkos and you can build power or live a relaxing life without the interference of the Marines.
Hell Moria didn't even do anything illegal. He just pissed them off vis a vis Luffy and even then that's another warlord's fault.