That is annoying, Chihiro. Though I think its fairly common among therapists--Dr. T won't fill out any paperwork either, just sends me invoices for each session (they're autogenerated with some program he has). I pay him up front, then use insurance for out-of-network benefits. He gets annoyed when my insurance company randomly sends him something in error. Like he'll text me a photo of it.

You're right about the potential for insurance fraud. The insurance company did call him once to confirm that I was in fact paying him the full fee up front (he didn't mind that call, because he understood what it was about). I imagine it would be possible to create a fake invoice to submit that says I'd paid, then get some of the money back. Or potentially, a therapist could conspire with a client to give the appearance that I attended more sessions than I did (and more than I paid for). But he has receipts of my payments (credit card), so he could prove I paid if needed.