The drones that are capable of launching those missiles are huge, and require airstrips to operate - we're not talking quadcopters here. Consider that e.g. Reaper - which can carry at most 2 such missiles - has a wingspan of 15 meters, and weighs a ton. Any such thing trying to cross the border would be quickly detected and shot down, same as any other warplane.

There is a different issue with small off-the-shelf quadcopters that are turned into "assassin drones" by adding a small shaped charge sufficient to kill the target if they manage to get close enough to detonate it point blank. But that's also something the Secret Service can handle fine, once they realized that it was a viable attack vector.