If people use it and are understood when doing so it is correct. "Each society," a group of people, "has their (own) laws." Therefore "their" arises out of the view that the collective noun "society" is made up of component people parts, adding a degree of personhood. In addition, the argument that "their" is strictly meant to be used as a plural pronoun has long been null and void.
This is a very enlightening discussion on the subject - http://ell.stackexchange.com/questions/49292/do-we-use-its-or-their-with-a-collective-noun.
Generally I support the use of "they" as a gender-neutral third person singular pronoun. English has no alternative, and it's been done for hundreds of years. However, I personally would not use "their" here because a society is a thing, not a person: an "it", not a "they". Most of the arguments here seem valid in at least some way, but I don't see any "their" supporters explicitly mention the support IN SPITE of a society being a thing. I'm curious where they stand.
John, lentalaga is wrong. "It's" is actually incorrect here because it's a contraction of "It is" (like, "It is the law" = "It's the law").
The Duo sentence uses the possessive "its (without the apostrophe), the kind that owns something (like your, his, her, my, etc).
"your law" = the law of JohnSmith
"his law" = the law of Señor Owl
"her law" = the law of Señora Owl
"my law" = the law of tessbee
"its law" = the law of each society
It's the law that nobody cares about" =
It is the law that nobody cares about.
There are two things which make its/it's confusing:
- The rule for forming contractions of pronouns followed by the verbs is or has requires that the pronoun be followed by 's.
he is - he's
she is - she's it is - it's
- The rule of possessives requires adding 's to a singular word to make it possessive: the cat's meow.
However, all pronouns used in conjugating verbs have their spelling changed to make them possessive rather than adding 's - except for it:
I - my
you - your
he - his
she - her
we - our
it - its
The problem here is that, if you try to impose both rules on it, you end up with ambiguity: it's the contraction would be spelled the same as it's the possessive.
So, to clarify the ambiguity, English grammar sets the contraction rule above the possessive rule, and it's is declared to mean it is or it has (as in it's got to be one of the stupidest rules of English ), and its is declared to be the possessive. There is no reason to give contractions priority, they just are.
The result, again however, is that "its" violate the rule on possessives. Unfortunately, the spelling has to be that way, because otherwise there would be even more confusion, and English grammar is confusing enough as is.