There is no requirement for Flight Attendants, or indeed anybody else, to justify or explain the contents of their baggage to anybody else. FA bags are present because they are required.
The fact is that recent developments within the LoCo industry mean that customers are encouraged to bring hand baggage only: Checked baggage is discouraged to speed turnaround and save weight. Given that space in the Cabin is finite, it is clearly unrealistic to expect that an in-finite volume of cabin baggage can be accommodated in the overhead bins - Some will have to be stowed in the other approved stowage, namely underneath the seat in front.
FA bags are not the cause of the problem, nor even a major contributor. However, because they tend to be labelled as such, they represent an easy target for the disgruntled.