Point taken about the marathon requiring special skills, but those 5 and 10k times are much much harder to get than a 2:22.
And those 5 and 10 standards are the equivalent of about a 2:15 marathon. So even if a 5 and 10k runner can't put it together at the marathon distance to get a 2:15, he could probably still manage a 2:22. So I'd say this is fair, and allows some fresh talent, right out of college, to run the trials.