Jonathan (rightly or wrongly) believed in the greater good in his sacrifice; the 'needs of the many outweighing the need of the one' if you will...
Let me rephrase my original response. The needs of the one was on Jonathan Kent's side, but the needs of the many were on Clark and Martha's side since they both wanted Jonathan safe. I don't recall Martha holding Clark back when he looked like he was about to rescue him. So "the needs of the many" thing doesn't really apply to his decision at all. Jonathan was just an idiotic coward who was willing to throw away his life because of something.... he didn't know. I guess it's a lot like the Prime Directive in Star Trek. We don't know what will happen if you save this culture from extinction, so why bother? Let them die Papa Kent would say.