Its so scary to sell your information, specially if you are about to send your passport information to other people. This information can be use in illegal or evil activities.
Many of those people are basing it on the argument that even if you dont personally sell your data, there are people that would sell it for you which we can see in the case of Facebook, but one thing that needs to ring bell in their ear is that it is better for someone to use it without their consent than for them to use it and which they can even make money if they find out that someone used their data illegally, they can sue for money, but to destroy ones self because of the little token, it is really not worth the risk.
I remember some months back that there was this website, that offered to pay people$30 for giving their data, and Oh my God, people were actually giving it and I wonder if the level of poverty in the world has gone that deep to sell what can kill one for just $30?