@SGRaaize:
Because he cares about Fishmen Island a lot and he went to save Whitebeard so that Fishmen Island could be safe from danger? I dunno, sounds reasonable
That's a pretty big jump. Sure, it's reasonable to conclude that he cares a lot about Fishman Island and his people, or that he would fight to protect it when and where he can, but to be near certain that he is duty bound to protect it for as long as he can? As a former Warlord and a bounty head, I just don't see how someone like him would find it wise to stay at Fishman Island forever, especially openly.
@Zik:
It stems from his entire existence being about the welfare of his people and the only reason he was in ID being due to a debt to a man who protected his island and his people.
[hide]Jinbei: The reason there is peace in Fishman Island is due to the help of your "Dad," Whitebeard. On the trip through the Grand Line, every pirate must pass through Fishman Island. When the Age of Piracy began, the island was soon wracked with violence, torn to shreds by the countless human pirates and the Marines that pursued them! Many fishmen and mermaids were kidnapped… sold... When we had all shut our eyes tight against despair, Whitebeard made his appearance. I'll never forget that day! Ever since then, Fishman Island was off-limits to anyone who would seek to run rampant The POWER he wields…! He protects many islands with the use of his name in this same way. This is not the kind of man who can be freely toppled, just because he's a pirate captain! SURELY the government is aware of what would happen on the seas if Whitebeard should die!![/hide]
Coming and going, doing what he can here and there, sure, but remaining there as its protector? As an undercover cop, he can't accomplish much. Look at Whitebeard and Luffy as examples; it's the name more than the actions that maintain order. Jinbei could put a stop to an attack, but his name could prevent it from starting in the first place. His name, however, attracts attention. I just don't see that being a permanent place he would stay. Likewise, why do people think Fishman Island will still be vulnerable and in chaos when Luffy leaves? Also, it's again worth noting that his concern wasn't explicitly for Fishman Island.
@Zik:
Something he was unable of.
@Chapter:
"When we had all shut our eyes tight against despair, Whitebeard made his appearance. I'll never forget that day!"
You know, Jinbei's ideals setting him apart from normal fishmen and all, I wouldn't be surprised if he wasn't a fighter or pirate at all before Fisher Tiger stormed Mariejoa. I mean his personality sort of clashes with his status as a pirate. We know from the above quote that he wasn't capable of helping or saving Fishman Island before Whitebeard, and the text suggests he was present when Whitebeard made his proclamation. I wouldn't be surprised if he was just an average fishman during those times, and that sometime after, being spurred by the treatment of his people, Whitebeard's actions, and the actions of Fisher Tiger, that he learned Fishman Karate and became a pirate (in no particular order). My baseless guess would be that he became Fisher Tiger's disciple ~15 years ago after the attack on Mariejoa.