first off, the ''artist'' would be easy to find as his or her finger would be dirty...and fingerprints might be easy to find.
It really works like this: IF YOU THINK that the plane is in jeopardy you have an obligation to stop the op (operation) until YOU are satisfied that safety has been restored.
The FAs will sue and will win. They will go on talk shows and make united look even more stupid than it already is.
Were those markings authorized by UNITED? OF COURSE NOT< so someone has tampered with the plane.
I saw the images on local tv news (our of the bay area) and it looks more like one face is oriental and one is occidental. Perhaps a message from one MX department to the other.
But how did the girls know? Did the F/O or someone tell them after the walk around?