I don't think it should have been stopped. If you watch, the ref jumped on the other guys back right when kimbo went down and then let it go a little longer. Also if you watch, the other fighter looked staight up scared when the fight started. He was backing up and jittery. Caught him with a defensive shot it looked like.
I might add that I hate kimbo, but in his defense, I think he would kill this guy in a rematch.
And I do think Shamrock pussed out too.