The modem is rented out by the ISP and it is their property -- this is not a clause in the contract that you signed. If it offends you then install your own modem.
I think there are legitimate questions: at which point would it become not acceptable for you? who decides what is blocked? should it be enabled by default?
Also if the ISP is in the business of "editorializing" what the user sees, maybe it becomes actually responsible for the content (no safe harbor). That also seems like a dangerous game for them.