Prostitution has existed everywhere for all time. Claiming otherwise smacks of exceptionalism and I don't accept it. Facets of it may have changed, but as long as there has been something worth exchanging men have exchanged it with women for sex.
That's pretty "first world problem" if Koreans are resentful of Americans, 37,000 of whom died not to defend their own country but South Koreans', because some girls ended up becoming hookers for them. If there was widespread raping and pillaging I'd understand, but if they were having sex willingly and were being compensated, then meh. I support the legalization of prostitution so I don't see it as some great evil. Much ado (and taboo) about nothing.
Also, the US imported hundreds of thousands of tons of food to feed the population in the late 40s, many who would have starved to death otherwise.
As far as aid, the US gave $1.75 billion in the period of 1953-1960. The UN's Korean assistance fund, in comparison? $120 million. That's about 15 times more aid from the US than the entire "community of nations." By the late 50s a whopping 70% of imports were funded by foreign organizations, which we've already seen were dominated by the US. That included essentials like food as well as raw materials necessary to help build the country.
America definitely invested in South Korea, financially. Did it taper off? Of course, and rightfully so. You invest the most when the nation is fledgling, not when it's getting its shit together. South Korea needed to walk on her own two feet eventually, and she did. But she would have starved to death or had been a slave before that was possible without both military and economic aid.