A thread at DigitalPoint forums named MSN SiteSearch TOS, discusses some of the language they use in their Microsoft Search Service Referral Terms of Use. One line reads, "You will not use the Service or the MSN Code in connection with any Website that disparages Microsoft or its products or services." That line is translated by the DigitalPoint member as "You have to be nice to us on your site." An other line the DigitalPoint member quotes is "A court may hold that a part of this contract cannot be enforced as it is written. If this happens, then that part will be replaced by terms that most closely match the intent of the part that cannot be enforced," which he translates to "If our contract is vague and unenforceable, use the next closest law."
In reality, MSN is just trying to protect themselves. Look at Google, they provide all these tools and SEOs sometimes abuse them and then complain to Google about it.