The item in question is in my hand....not the blonde but the little metal cart.

Basically it folds out and has a telescoping handle and my sister straps the carseat on it so she can pull my nephew around the airport and doesn't have to deal with gate checking the stroller.

Flying on SFO-DEN (Y) DEN-MTJ (Y/F) MTJ-DEN (F) DEN-SFO (F).

On DEN-MTJ she actually carried it on and was behind me and said "The FA said it can't go in the overhead."
I figured they may have glanced as it and it wasn't collapsed or something. So I put it in the overhead bin. On the PA there's a "You have to take that down." At the same time a mechanic(-ish) man was walking up and said "They don't like those in the overhead bins because they may fly out and hit someone."
...vs a suitcase that probably weighs 30# and is put in the wrong way?
So I put it under my feet and it sticks out a little. The FA comes by and says "That can't go there."
...but you just told me to put it there (I may have been poking the coals at this point).
"You'll have to gate check it." This small, flat cart. I said sorry no one on our previous flight mentioned it.
"Well they're new."
At this time the GA came on and gave me my CPU upgrade. Once I was moving to F the FA said "Okay you can put it behind the last row of first class" (A319).

On the flights home no one said anything in the overhead and on DEN-SFO the FA actually opened the closed bin for me as I had it in one hand and was reaching for the latch.

We're going to be flying SFO-HNL-SFO and SFO-PDX-SFO but I figured this FA was just in a mood. I'll probably also try to book the 777s for HNL so she doesn't even need to bring the carseat for him.

Not the end of the world but tired of reading threads crying about no more shoddy 747s flying or people not asking for a PDB and getting upset about not getting one.