That is correct, that is why if u plan to sell the phone it is best to restore and don't jailbreak it.
I once sold my old iphone4 and it was JB, I had the same problem. To make matters worse, the people's home internet connection were I went to sell it was not working fast enough and it was taking an eternity for itunes to download the IPSW file (which can be in excess of 400MB in size). In the end I gave up & ended up giving the phone to the new owner with all my pictures, I manage to painstakinly delete all my contacts, some calendar entries, but I felt like I had left too much confidential info prior to selling it.
The next time I sell my 4S, I'll make sure I restore first.