Post-race workouts by itself are nothing bad. Tinman prescribes them after unimportant races, to keep his "only 2 hard days + LR a week" mantra going and get in some threshold or CV work or speed work that was missed on the first hard day.
Salazar does them to teach the body to still keep going and run when everything hurts and is at its limits - think about Hassan hammering down the last lap in a 1500m race, everyone is suffering by that point but the hard post-race workouts put her so often in this situation that she has an advantage.
Now the BIG problem with what Salazar did with Mary Cain. Ever since she heard about Salazar, she was looking up to him. He was the most successful coach in America, and she wanted to do nothing more than make him happy. She trained very hard, did everything he asked her to do and she wanted to be the perfect runner student. Then, in Boston indoor Jan 2014 she beats NCAA champ Abby D'Agostino and Jordan Hasay by running a 4:24 mile as 17-year old girl and setting a BIG PR in 1500/Mile. What does Salazar do? Instead of finally encouraging her and telling her how fantastic her performance was, he sends her on the hardest post-race workout anyone can imagine together with NCAA runner Hasay who had much more mileage and training in her legs at that point. The post-race workout - 4 mile tempo in 5:30 pace, followed by extremely fast 600-400-300 and then an all-out 200 in 27s, made her vomit and destroyed her.
But the biggest damage was done mentally - even the 4:24 mile, running her heart out and beating seasoned NCAA runners was NOT good enough for her coach, instead of celebrating the race with a nice post-race meal or something he sends her on the track to do a superhero workout. She was mentally broken after that day. She was starving to get to this level, suffering through insane workouts with Hasay and Moser, then even manages to beat runners she was not expected to beat and instead of finally allowing her to eat a pizza or burger Salazar puts her on the track to do a "best post-race workout in her life". That would break most 17-year old girls mentally, and I am not surprised she was never the same anymore after (after this race, most of the problems started in early-mid 2014).