The government controls EVERYTHING to do with GPS, which includes WAAS.  The only thing that can shut down (or, more accurately, make useless) the Deere corrections signal is the gov't if they decide to degrade the base signal that the Deere corrections work on.  Read the following article on Selective Availability and make sure you note the difference between GPS signals and DGPS.  There are a number of ways to provide a differential signal like WAAS or the Deere version.  WAAS and the other corrections work on the base signal which the gov't controls.  The gov't isn't going to screw up the base signal for everbody BUT Deere if there's a need to do that in time of war or emergency.  The only way JD could do that is to launch their own satellites and hope the gov't doesn't decide to jam them or shoot them down if they become a security risk.

http://www.ngs.noaa.gov/FGCS/info/sans_SA/