This is also a violation of FAA safety regulations. I recall when they put those regulations into place - children have to travel in a separate seat, usually with a booster seat. In fact, parents are required to file for a specific exemption if they're traveling without having a child in a safety restraint device.
https://www.regulations.gov/document?D=FAA-2007-0001-0001
The commercial airline industry saw it as a business boon, requiring parents to purchase an extra seat for infants and children. It's not a hard and fast regulation, per se, but the airline companies treat it as such. Unless, they can resell your child's seat.
https://www.faa.gov/passengers/fly_children/
Yeah, this is a blatant violation, they're going too far here. United should be fined. Safety regulations exist for a reason. That the passenger feared for their safety if they complained in order to protect their child is beyond the pale. Totally unacceptable that passengers are put in this situation.
As consumers, we all have rights. Call your senators and congresspersons.