The point is to get people to open the letter, yes. Having people open the letter who won't actually respond has zero cost to the advertiser. The letter itself is a sunk cost, so trying to push people away from opening it is not beneficial. By trying to entice them to read, in the worst case, they just throw away your letter a little bit later. More people opening the letter is not a negative like being deluged with thousands of false leads would be for the Nigerian scammer.