Short answer: air fresheners themselves aren't illegal anywhere in the US — but how and where you hang one can be, depending on your state.
The actual law
Several states — including California (Vehicle Code §26708), Texas, Arizona, Pennsylvania, Illinois, Minnesota, Virginia, New York, Louisiana, and Florida — have laws against hanging or attaching objects to your rearview mirror (or otherwise obstructing the driver's field of view). These laws aren't about air fresheners specifically; they cover anything hanging there — parking permits, graduation tassels, GPS mounts, and yes, air fresheners.
What this means for you
The classic tree-shaped freshener has become a running joke about this exact law, but in practice it's rarely the sole reason someone gets pulled over — it's more commonly cited alongside another violation. If you're concerned, hang your freshener low and to the side (not centered), or use a vent clip instead of a mirror hang — vent clips aren't covered by these obstruction laws at all. Rules vary by state, so if you're a stickler for the letter of the law, a quick check of your specific state's vehicle code is worth it.
Bottom line: it's not the air freshener that's illegal, it's obstructing your view — so hang it smart (off to the side, not dead center) and you're in the clear in every state.
Sources: California Vehicle Code §26708; state DMV/vehicle code summaries.