I think it's more about the literal action of building on sand. It erodes quickly and hence requires upkeep (dredging from where it flows to, and restocking where it flows from). Sand also has a nasty habit of liquefying during earthquakes, destroying buildings at their very roots. Although the Korean peninsula as a whole isn't at too great a risk of tremors[0], it's still a risk that they're taking.
[0]http://geology.about.com/od/seishazardmaps/ss/World-Seismic-...