You’re right-fingernail painting should only be done under safe circumstances, such as when the painter is driving in heavy traffic.
If everyone paints gets to paint their fingers and toes; what do you suppose the plane will smell like? And, if 5% of these folks spill the paint container due to air turbulence, what do you suppose the seats, carpets, cabin and trays will look like?
Would you are to sit in a seat, that is wet from fingernail paint, because some woman decided to pain her nails, and the container spilled? Or, would you like to be the person who gets left behind, because the seat you were assigned is ‘temporarily’ out of service because of an obliviot?
The attendant asked her to stop painting - and the passenger decided that she was entitled to continue - so we’ll let the courts decide. I fly a bit more than the average person - about 2x a month. I’ve never seen someone rude enough to paint on an airplane.