Why was XABI ALONSO banned from publishing a goodbye letter to Liverpool's fans?

It has been revealed that Xabi Alonso may have been prevented from writing an open-letter to Liverpool's fans when he left the club. Given the Spaniard's contribution to the club, I find this to be very disappointing.

Both John Arne Riise and Luis Garcia wrote open letters to Liverpool fans when they left the club, both of which were published on the official Liverpool FC website.