Because July 1st in when the window officially opens for you to able to register players. I think.
You can still sign off and finalise a deal before then though, he just wouldn't become a Newcastle player until July 1.
We've already announced Atsu before July 1 so no reason we couldn't announce this soon as it's done and dusted, the article will just say "Lejuene will become a Newcastle player on July 1".
EDIT: We announced Cabaye on the 11th of June 2011, For example.