Scientific explanations, which explain the behavior of an actual object by relating it to the behavior of an ideal object, don't usually involve a normative element. It's not as if we think that inclined planes should be frictionless, or planets should be perfectly spherical. These ideal models aren't somehow better then the actual objects in question, they are just easier to think about.
I wonder if psychological explanations of actual human behavior by relating it to rational human behavior ("the price rises because if everyone was a homo economicus with this set of beliefs and desires they would..." "actually, getting a beer is what a fully rational person with Jim's beliefs and desires would do right now..") are just instances of this. If they are, then the normativity makes no difference to the explanation. The idea that one ought to be rational (assuming there is such a fact) plays no more role in the success of the explanation than the claim that inclined planes ought to be frictionless plays in the success of the ordinary physical explanation.