You should team up with the lawyer in the other threads running around taking pictures at airports. Sue everybody.
United, as shown in quite a few threads here, is very sensitive about their people getting their picture taken.
Their policy is pretty clear:
Also, unauthorized photography, audio, or video recording of airline personnel, aircraft equipment, or procedures is always prohibited.
The constant need people have to photograph the United Employees probably drives them nuts. The correct answer should be "sorry, I didn't realize it was not allowed"and to move on.
Not to challenge them and piss them off even more.
I know, I'm in the minority, but nothing good will come of the arguments with United employees. They have all been told no pictures. The customer is not going to change that.