Why would the Jeju Koreans want to return to Japan? That would probably start more violence on the island and threaten the ROK position. And I don't see how giving it to Japan is going to stop conflict, considering the DPRK will continue to claim it as an integral territory of Korea and further be able to use it as proof that the United States and capitalist West are firmly on the side of imperialism and exploitation of the Korean people given that they just handed back Korean land to the former imperialist oppressor Japan. Given DPRK's constant use of nationalism in their propaganda, they'd be able to score a huge propaganda victory over their own people, the Jeju Koreans, Koreans abroad (mainly in Japan regarding Zainichi Korean groups), and probably other peoples in the colonised world too.
A lot of post-colonial countries became independent with 300K or less people although in the early 50s there weren't many independent countries with that many people. In any case, a significant number of refugees might be able to escape there to increase the population further.
I didn't say the Jeju Koreans would want to return to Japan, just that the US might decide that the reconstructed Japan would be the best place to put them to ensure they'll be part of a strong and anti-communist state.
And look, if you know the Aland Island situation you know that those people get such preferential treatment in terms of lack of taxes and local cultural and political autonomy that they don't want to go back, so if we assume there is some autonomous Principality of Jeju/Saishu that is thoroughly Korean but protected by Japan then I fail to see how much hassle this all peninsula DPRK could cause.
Again, I'm not saying this would be the best thing for the Jeju people just that the US might decide that this the best way to avoid constant conflict. Imagine the Gibraltar fishing and oceanic disputes but on steroids if both this ROK and DPRK claim all of Korea. If this Jeju state is instead thoroughly dedicated to being independent (there's a degree of cultural distinctiveness that could back this up) we'd likely see less conflict if one side would rather annex one area eventually but doesn't feel threatened by a government-in-exile. Such a DPRK can deal with this Jeju state on realpolitik terms without worrying about being a base of operations for peninsula spanning anti-communist rebels and operations.
And how many of those post-colonial countries nearly fell or did fall to communism or were under threat of doing so?
And on the note of being seen to support imperialism, Japan is still under occupation and could easily have Jeju autonomy built into their constitution like the OTL Self-Defence clauses. Besides, Japan still had a thriving left at this point that would hardly resurrect imperial discrimination and would make it hard to any of the right/far right parties to do so. Jeju could also be make like the Ryukyu protectorate and annexed into Japan or made re-independent much later such as '72.
And even if we assume this decision turns out to confirm all the worst fears of the Jeju Koreans, that doesn't mean it might not seem like a good idea in the moment and it doesn't mean that the US might not enforce it to ensure a strong anti-communist presence in the East.