Explanation:
Here’s the ruling from AP:

Use a hyphen whenever ambiguity would result if it were omitted: The president will speak to small-business men. (Businessmen normally is one word. But “the president will speak to small businessmen” is unclear.)

Indeed. The reader might think that the president will speak only to diminutive businessmen. One envisions a giant ruler at the door and a sign to the effect that you can only attend if you’re “this small.”

By hyphenating “small-business,” we make it clear that “small” refers to “business” rather than “men.”

Of course, the whole issue could — and usually should — be avoided by using “business people,” which isn’t spelled as one word anyway.

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