Version 5.0 or later
No longer restricted.
Version 4.5.8 or earlier
The reason you aren't allowed to do this is because of the API used. The API determines what string to bolden and if they are the same then the same word will be bolden.
This is would happen if you were able to post with username containing/matching display name.
Not very faithful towards the original app so this restriction is added to stop this from happening.
Customer support service by UserEcho