Nethys's tenants (except "don't free Rovagug" and not granting magic to the uninitiated it seems) are explicitly self contradictory and up to the individual, and one of his divine servants is from an species made of Law and Good. Why can't he have LG, CG, LE or CE clerics?
Is there just Paizo never giving an exception to the "one step" rule for Clerics, or is there some fluff that justifies it?
I think it's just a rules thing. However, you could probably interpret it in that someone who embodies that alignment in the way a cleric would just wouldn't follow Nethys. There's too much room for good/evil/chaos/law for them to truly dedicate themselves, and so they either seek another god, or slide into one of the neutral alignments naturally. Worship is a two way street, in a sense.
Nethys is pretty high on my "I hope I can find an occasion to make a cleric of this" list. I really like the idea of the duality built into the concept, with the Protection and Destruction domains, focus on theurges, etc. The same logic that's behind Druid alignment restrictions, and that Nog hinted at, seems sensible in this case.
That certainly doesn't stop those on the corners from worshiping Nethys, and there's plenty of good, characterful reasons for doing so, but being a Cleric means being held to a different standard than the average schmo.
Nethys values magic above all. If you value the common welfare (good) or personal freedom (chaos) strongly, he can accept you. But if you're caught up in the complicated eddies of opposing tyranny (LE), promoting the personal freedom of everyone (CG), he doesn't have time for you, and you probably don't have time for him.