A curse can be looked at as just the absence of a blessing. Orlanth blesses his followers with hard hats, with beef and lamb and venison, with not needing to eat each other. Issaries blesses them with speech.
The way I see it is that everyone was cursed during the Great Darkness. This community of Thanics survived (they are stronger, more skilled, more stealthy, know more, have horns, have jet black skin) thanks to the Gifts of Than. They do not recieve the blessings of Orlanth, but on balance, the gifts allowed them to survive. To prove it, they're here.
Everyone was miserable, huddled in darkness and fear during the Darkness. Than worship, headhunting, cannibalism, stealth and atrocities allowed these cowering, fearful, people to survive. The Geasa are a throwback to that. They are rejecting the blessings of the Lightbringers rather than being cursed.
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