Those are actually ancient yardage markers believe it or not. The markers are over 300 years old and since they are historical they
can't be removed.
They do cover them during play because they are a safety hazard due to balls smacking into them off the drive and ricocheting into other fairways and walkways.
You can't hit them yet, but it is a subject of debate... *evil grin*
*edit* Found this interesting article regarding the markers. Scroll down to the August 5th entry almost all the way down the page. http://www.lpga.com/content_1.aspx?mid=2&pid=12017