You may have noticed that paragraphs 15(b) and 15(c) both have the word “tolls” in them. Paragraph 15(b), as you mentioned in your question, gives regulation-making powers vis-à-vis tolls, but you may also have noticed that it is to ensure the efficient flow of traffic. In other words, it's not to come and regulate things just for the sake of regulating; it is regulating within the circumscribed circumstance put in this provision, which is to ensure the efficient flow of traffic.
As long as tolls imposed by or charged by bridge operators don't impact the flow of traffic, then that power cannot be exercised to regulate.