The official policy printed in the Hemispheres magazine is as follows:

"The use of still and video cameras, film or digital, including any cellular or other devices that have this capability, is permitted only for recording of personal events. Photography or audio or video recording of other customers without their express prior consent is strictly prohibited. Also, unauthorized photography or audio or video recording of airline personnel, aircraft equipment or procedures is always prohibited. Any photography (video or still) or voice or audio recording or transmission while on any United Airlines aircraft is strictly prohibited, except to the extent specifically permitted by United Airlines."

There is no FAA regulation involved. It is simply UA's policy. But the best way to think of it is like speeding. If you're only going five miles an hour over the posted limit, 99% of police won't pull you over. Same goes for discreet photos of the seat, cabin, or food. But if you photograph the FAs as they work, you're more likely to be confronted.

Then there's that 1% of FAs who are either looking to make trouble or who have a personal dislike of being photographed, even innocently. That seems to have been what happened to @MatthewLAX.

In sum, as with so much in life, YMMV. Be discreet, and don't point your camera at any FAs, and you probably won't have any problems.