I think there are some other factors at play here. Think about how demoralizing it must be for Nadal's opponent to know that he won't act in the 'rational' way

I think there are some other factors at play here. Think about how demoralizing it must be for Nadal's opponent to know that he won't act in the 'rational' way that you outline - instead he will keep trying to come back from 40-0 and sometimes succeed. Also, if you adopt the attitude that you're trying to win in the long-run and so it's better to concede this game to conserve your energy, it becomes easier to use that as an excuse to be lazy.