That's something new to me. Around here one of the biggest issues people have in bathrooms is mold, second is moisture damage to paint and drywall. Putting the fan on the same switch as the light helps mitigate the problems, especially in rental units where bathrooms tend to be smaller. I would argue that a timer for both would be a better solution all around, but in an argument against Federal regulations I would just be banging my head against a really big wall...
I know this was an issue in a rental unit I knew about. The owner had us wire the fan both on a timer and on the light switch because of the mold and mildew being a problem. Some tenants even tried to blame and sue the owner/property manager, but it turns out they just weren't turning the fan on because the fan was on a seperate switch from the light!! Not only that, the Department of Health says that even if they turned it on while they showered, the fan needed to stay on at least 10 minutes after they are done because of the build up of steam. So the few that did turn it on, didnt leave it on long enough.
So now the fan comes on when they turn on the light in bathroom wheather its needed or not LOL. Also, the fan (not the light) comes on and runs for 30 minutes twice a day from the timer wheather its needed or not LOL. But in most cases its needed
This took care of the mold and mildew issue, the owner is happy because no more 'threats' of lawsuits from his tenants about this mold and mildew when it was really the tenants fault!!!
Now back to the topic at hand
in our area you are required to put the light on a gfci protection if it has exposed metal parts and intstalled in the shower or within 5ft of the water line of the tub.
" All luminaires within an enclosed shower area or within five feet of the waterline of a bathtub must be enclosed, unless specifically listed for such use; these luminaires, with exposed metal parts that are grounded, must be ground fault circuit interrupter protected."