Yeah good question! Reading that story, I understood first that rats like sweet things and hate bitter things. Second that rats are usually willing to take Hobson's choice, and the point of the article being that most scientists don't realize that's what they've actually been testing.

I understand that heroin withdrawals are pretty terrible, I've even heard said that "you'd do anything" to make them stop, but they can't kill you in a sort of way that alcohol withdrawals can.

It sounded from my reading more like toward the end of the experiments though it was becoming clearer that the rats in Rat Park genuinely did not like the effects of the drug on their normal lives, and when they had the other option of socializing, they were only still taking the drug to get at the delicious sucrose.