This is what stands out for me. Of course they are entitled to a different opinion, it's that they let her be hung out to dry.
A few people have commented that no one owes their previous employer anything, can't not speak out against them etc but they weren't ever established actors. By giving them these roles, JKR 100% had a hand in the privileged (at least in the monetary sense) lives they now lead, and they have both proved to be not very good actors as a result.
So that's what it's quite galling for them to have behaved the way they have behaved, difference of opinion or not.