I did make a mother unhappy on a flight where I was preboarded with medical issues and when she got there I was already reclined in my seat and she could not fit her rearfacing carseat. she ended up front facing seat unhappily as that was only option FA give as it was full plane. it was a young one not sure how young. FA would not try to reseat all 3, offered for mom and baby but not the whole row
If the baby was under 22 pounds, the FA was absolutely wrong to have the parents turn the seat around, and I wouldn't have done it. Children are allowed - and should according to the NTSP, FAA, AAP, AMA, etc - be properly restrained in something designed for their age and weight. For a baby under 22 pounds, that is a REAR facing carseat. The FA should have tried to reseat the entire party, which shouldn't have been hard - I'd bet a million dollars there was a row who would have easily done a 3-for-3 swap. Most people don't really care if they are in row 22 or 21.
(This is different then the poster who asked about her 17 month old, who I'm guessing is over 22 pounds. At that point, you can think about issues of courtesy.)