In the article they mentioned later that in gift-giving cultures receiving a gift make one indebted. They are rejecting the offers, not because they are too small, but because they are too big. The people offering as much as 60% are doing so to gain favours.
Probably there is some heuristics at work, which gives the intuition that a too good split is somehow dangerous. (Similar to how people who are afraid of the dark try to avoid both caves and cellars. )
