Still, it is completely surrounded by Croatia's claims, and located in the narrow Bay of Mali Ston that Croatia already has a bridge over, so if Bosnians get naughty again they can just turn their sea access into a lake just by dumping enough sand/clay/silt from the existing bridge.
Other 2 answers describe current situation, but the origin is much older. In 1699 Repubublic of Ragusa (now Dubrovnik) ceded Neum to the Ottoman Empire, to prevent land attack from Venice, as Dalmatia was part of Venice that time. Than BiH and Croatia just inherited the borders.
Most living in Bosnia along the Croatian border in the south-west are already ethnic Croatians, and they cross the border unimpeded to Croatia for all its benefits. Those that are not ethnic Croatians are likely ethnic Bosnian there, who cross as unimpeded as well, except they likely give less shit about it because they are just visiting their relatives or enjoying their EUR-bought or built homes during the summer but then go back to Germany to work for the rest of the year.