II think the "not for cats" has to do with people trying to "guess" at how much dog stuff to use on a cat. The cat product has EXACTLY the same amount of fipronil and methoprene as the Frontline Plus for dogs. It's just easy to overdo it on cats if you (a) have no idea what you're really doing (which the average bear doesn't), and (b) since you don't know how many "drops" are in the dog size you have, you really don't know how many drops to put on a cat unless your veterinarian has told you, and (c) it's easy to overdo the drops since you have to squeeze to get it out of their little plastic receptacles.
It IS okay to use on cats (Frontline Plus and equivalents). The label dose for cats and kittens OVER 8 weeks old is 0.50 (half) a milliliter or centiliter (CC) every 4 weeks. If you have the little tuberculin, 1 cc syringes, that usually come with a 25 gauge needle, fill to the 0.5 ml/cc point and apply all of it at the back of the neck directly on the skin, after parting the coat (a bit higher than where the shoulder blades start). You don't have to spread it out in multiple spots for kitties. If you have kittens that are underweight, sick, or less than 8 weeks old, and they really need it, use one drop per pound. I generally tell people NOT to use it on kittens less than 3-4 pounds (use a permethrin shampoo instead). If the kitten is 3-4 pounds use half as much (0.25 ml) and save the rest to repeat in 4 weeks. After that, they should be big enough for a full dose if healthy. A heavy flea infestation can kill a kitten (less likely with a puppy, but can happen with them as well if it's a toy/small breed). An overdose of any pesticide can make an animal sick. Also, if the cat gets a mouthful of fipronil, it will foam at the mouth like crazy because it apparently tastes really nasty. Don't apply it to multiple cats at once--lock one up at a time until it dries in about an hour or two. It usually is not harmful when ingested by an adult cat, but you need to watch them for the day if it happens (happened to me the very first time I used it, right after it came out, and I had to call their customer service line!). I had a cat that could spin her head around almost backwards, and I saw her licking it. She turned out fine, but she was a healthy, good sized cat. I kept wiping her mouth for a good 20 minutes before she stopped salivating.
Hope this helps those of you (many) who also have kitties. You just have to be careful! Also, if you have a really fat cat, I'd add a bit more. If your cat is a 15 pounder or heavier, you might consider adding an extra tenth or two of a cc (total 0.6-0.7 ml). A 20 pounder might need close to a full cc (0.8-ish). There is probably a weight range on the back of the box (I don't recall offhand since I'm retired). Check the back of a cat box to see how much 0.5 cc is intended to treat. Unfortunately, the online stores don't often show images of the back of the box.
Also, if you are going to bathe a cat, do it at least 2 days before or 2 days after applying Frontline or you'll strip off too much of the skin fats that help distribute it.