This happens all the time. And the other airlines also do it. In my opinion, she escalated the things by going to the social media to blast Delta. The airlines was acting within its TOS when they cancelled her seat.
I think what her point is, she has long legs and paid for the preferred seat. She is 6 foot tall or something with longer legs than normal women. Iam also 6foot tall and it is incredibly uncomfortable to sit in a regular seat on a plane usually.
This other person who took her seat is a small person who doesn't need extra room. She doesn't look to have any handicaps or issues
Appartently this woman wanted to sit next to her family, which isn't always possible and in the grand scheme of things they made someone else give up their seat who needed it more.
Even if it wasn't ann coulter, people with long legs or tall people who pay for a larger and more comfortable seat shouldn't be forced to give it up to a small person.