Two words:
Kinetic Energy.
If 200 pounds hits a jet at mach 2.6 (incoming rocket speed for SM-II series' slowest rocket against a stagnant object), you'd be screwed either way. It detects if its circuits are working as it is flying. If it notices that it doesn't have the ability to explode, it will boost and aim for the heat signatures (engines) In such a short time, the pilot would be unable to notice this small change and would lose his engine. Thing is, SM-IIs work off of more than 5 targeting systems at once.