Despite specifying what they did, it's still framed in a negative way
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.