I could see this as crazy frustrating.
I just received my credentials and finally got everything figured out for candidate entity management. I do NOT have a TOS to approve when retrieving auth code.
Notice on this page: http://developer.bullhorn.com/articles/getting_started
("Get an authorization code", Step 2) - They imply that there is ALWAYS a TOS to approve - this is not the case - it has to be enabled. I confirmed this with support via Chat.
I'd open a ticket with them and tell them to disable it if they cannot provide a way to bypass.
Alternatively, use the cURL command they provide - thou, I suspect you've tried that already.