To support Ady's point about the cloud car escort-- do not forget that Vader, Boba Fett and Lando are all in cahoots about the Millenium Falcon's arrival. So, while the cloud car laser shot was a show of belligerence so that Han and Leia would not be tipped off about the upcoming trap, it only makes sense that since all parties are already aware of the MF's lack of hostile intent, that they be allowed to land in a patently unguarded fashion.

You could even say it was one of many reasons for Leia to say -- "I don't like this....." after they landed.