Oh I see what you are saying now, anyway like I said it isn't but preaching itself varies. If you go to you tube and see some videos you will see that not every preacher says the same stuff. While one may mention the clean slate upon conversion another might not even bring it up at all unless asked. That is why I do not think it is necessarily wrong. Remember converting to any religion is actually a huge life choice and so the person converting will undoubtedly have questions concerning said conversion, I will give you an example from my mosque there was a white girl who wanted to convert but she told the Imam that she had committed many wrongs and she also had tattoos. Now she looked up online and saw that tattoos are forbidden amongst Muslims and was concerned that she may not be accepted. Then the Imam told her about the clean slate and that she has nothing to worry about she can keep the tattoos because getting them removed is costly, painful and she had gotten them before she became Muslim. So I mean it really depends on the interaction sometimes it comes up sometimes it does not.