The problem is that they consider common forms of birth control to be abortion -- namely IUD and the pill which work, not just by preventing conception, but preventing implantation. (http://tpmcafe.talkingpointsmemo.com/talk/2008/09/probirth-control-sarah-palin-n.php) That way they can sound like they are pro-birthcontrol (or actually neutral on birth control, you won't see them actually advocating it anywhere) while actively working against most forms of it.