And they're not? They may not all participate in contact classes, but what's so different about the punches and kicks that they are doing than what a "fighting art" would teach?
MJS already pointed this out, most cardio classes do not actually teach the correct way to punch, kick, etc. Performing ballet-like movements does not make what you're doing ballet unless it conforms to the principles of ballet. Ok, so you're punching, but does the instructor tell the students and enforce not locking the elbows? As far as I know, no MA has locking of the elbows in punching. Cardio MA classes can be teaching a part of a fighting art, but oftentimes this is not the case.
On to Cardio Tai Chi...
Urban Tai Chi
“Explosive, energetic & fast-paced” - Fitness Magazine
This video gives you a challenging aerobic workout with the focus and control of traditional Tai Chi. The cardio routines blend smoothly with the Tai Chi movements to open your body's energy paths and give you a sense of power and grace. This is truly a unique video that will challenge your body with an excellent workout. Giinette Desiauriers is the host of the international TV show "Kardio Knockout" now seen in 21 countries with over 28 million viewers.
from this
site
While Tai Chi is not just a slow dance, it is practiced slowly in the beginning so that the student can practice proper body alignment. Imagine trying to play flight of the bumblebee on any instrument full speed without practicing it slowly at first. Unless you're a master of the instrument, the music is going to be a mess, and while it may resemble the flight of the bumblebee, it is not the song. I haven't seen this video, or attended any cardio Tai Chi classes, but I'm curious to see if they actually teach and enforce the underlying principles. Just because it looks like Tai Chi (and only then to an outsider's eyes), doesn't mean that it is Tai Chi. Calling it Cardio Tai Chi-like dance/exercise would be more appropriate.