[Galadriel55]

I think that it will help if we look at the curses' opposites - blesings. There are some that are, well, legitimate blessings, like Varda hallowing the Silmarilli. Others are merely respectful, or wish good luck - eg the Dwarves' "may your beard grow longer".

Same could happen with curses - a real curse versus an "I wish you all the worst" sort of thing. They might sound the same as curses, but their respective opposites are totally different.

For further clarification: it's like the difference between have to and must. They sound like the same thing. However, their oppisites, do not have to and must not are quite different: either you CAN, but it's not manditory, or it is prohibited.