While it is true that value scales are subjective, I think the “rational” descriptor is in there to suggest that everyone acts rationally according to themselves. In other words, the thief who steals your car most likely did not do so because he’s some sort of unthinking brute who merely acts out of some sort of animal instinct. He stole your car because he wanted a car, and in his mind, that was the most effective way to obtain one. Most people might not consider that to be rational, but we can infer from his actions that, at the time at least, he in fact did consider that to be in his best interests.
That’s what I got from reading Mises, at least.