Maybe because using her (not as a punching bad) but as a way to vent and express and explore your anger is REALLY OK. Maybe your T is saying, you need someone who is safe to displace your anger on at the moment...and she is willing to provide that for you. It doesn't mean that it will always be OK. Maybe she is thinking once the anger has been acknowledge and expressed to her, the two of you can work on finding ways to change the situation that makes you so angry.