For US advertisers Yahoo have recently changed their terms of service for the Yahoo Search Marketing the change basically states, Yahoo can:

• Create Ads
• Add and Remove Keywords
• Optimize your Account

Excerpt from TOC:

In the U.S. only, for those advertisers not bound by an Insertion Order, we may help you optimize your account(s). Accordingly, you expressly agree that we may also: (i) create ads, (ii) add and/or remove keywords, and/or (iii) optimize your account(s). We will notify you via email of such changes made to your account(s), and can also include a spreadsheet of such changes upon your written request. If you would like any of such changes reversed, please reply to such email within 14 days of the change(s), and we will make commercially reasonable efforts to reverse the change(s) you specifically identify. Notwithstanding the foregoing, you remain responsible for all changes made to your account(s), including all click charges incurred prior to any reversions being made. It is your responsibility to monitor your account(s) and to ensure that your account settings are consistent with your business objectives.

Note, "you remain responsible for all changes made to your account(s), including all click charges incurred prior to any reversions being made" - so Yahoo can make changes and you are responsible for them and the costs that come associated with those changes!

Crazily also, this is opt-out too!