The time period where they drank the pee was not actually labelled, but somewhat before the time period they're in now - probably around a hundred years before.
But your point stands - the recycled pee was the only potable water in the area - and Cable had already succumbed to dehydration (which is why he was unconscious and Hope had to find the water), so there wasn't any time for squeemishness about the source of the water, as long as it was clean.