The experiment was about whether rats can learn to drive a car. They can. It doesn't matter why. If they learned to talk to get access to sugar, we wouldn't go "yeah, but it's sugar", we'd go "rats can fucking TALK now?!".
But would they be jumping up and down when the dude came into the lab if they weren't looking for that next hit?
Or were they starving those poor creatures? I wouldn't doubt it.
I'll hazard a guess that they weren't jumping up and down because they wanted to take another spin in the new moving environmental element.
Or did they remember that when the dude came in, they could take the new thing to get a whole Froot Loop? Perhaps.