Yes, they are trying to develop a relationship with them where the person with BPD feels they can trust their therapist. I understand how you feel. I got in the middle a few times because they prescribed drugs that were abused. It was a mess. But that's the reason- they want their patient to trust them.
Herodias hit the nail on the head.
When my stbxh was in therapy. I was happy to turn him over to the therapist. He tried to bait me with little jabs about things they spoke about in therapy for a reaction. Not my business. It infuriated him to no end.
I began to detach. I was no longer his caretaker, problem solver or parent. After many months, the therapist was his target and then, boom! He wasn't going to therapy anymore.
I initially felt as you do, Concerns; like he was being allowed to lie his way through it but I trusted the therapist's ability to see through his bs.