If reality hurt's their feelings, maybe they should change their ways.
No one disagrees with you in principle, but there are realcitrant social practices punishing this kind of feedback. It is always taken as a grudge rather than an observation, and this makes you less attractive as a hire.
That is to say, (former) employee doesn't speak ill of (former) employer and (former) employer doesn't speak ill of (former) employee. It's how society keeps itself oiled.