Why are cool air intakes illegal for a 2006 mazda 3s, but not other years?

i bought one off of aem's website and i was excited about it, but then i researched it a lot more looking for exact hp increases for my car to get an average of what it brought a stock car up to. as i was searching i realized i read the website wrong and it said that it was illegal in like 7 states including california. i am still going to put it on, and just replace it with the stock one before a smog check. but why is it illegal for my car and not something newer and more powerful like a honda civic si?