My therapist always walks me to the door, opens it and says goodbye or see you next week. (I don't thank her because why on earth would I thank her for doing her job?)
If she suddenly walked toward her desk and turned her back to me in silence, I would feel hurt. I would interpret it as the therapist being annoyed or fed up with me. So I completely understand where you're coming from.
It's also pretty rude. You're right that you shouldn't have to ask her to do a pretty obvious thing (saying goodbye). Still how is she going to know that it's upsetting and hurting you if you don't tell her? Talk to her. Mention how it makes you feel. If she's a good therapist she'll listen.