We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
Did you know that when its first appeared in the late 16th century it was written it's? Well, it was - because it's represented pronoun it + possessive inflection -'s.
Even though the apostrophe would gradually fall out of use (probably by analogy with his/hers), it's remained in common use into the 19th century.
It's the legacy of our eccentric rules for the use of the apostrophe, which are just confusing enough to create doubt in those who are not all that sure of their written grammar and punctuation.
It should simply be The Smiths but the absence of that apostrophe creates an element of doubt among those who vaguely recall "something some English teacher went on about long, long ago".
So better to be safe and stick on an apostrophe just in case!