First, let's start with your idea that Israel has the right to shoot at them while they are doing it. This right does not exist. When a nation fearing invasion puts up strictly defensive barriers on its own territory, it is exercising a fundamental right and no one has the right to interfere. If the measures had an offensive nature, that might make it another case, but they don't. Next, the UN may have a group speaking out against laying mines, but that is just a group; the UN as a body has never ruled on this.
Law expanding definition of terrorist activity passes first Knesset reading (Haaretz)
from the article: IDF troops kill Palestinian in clashes at Gaza border