I'm guessing that when large corporations sign agreements with each other, they don't include the clause "we can change this agreement at any time". Only powerless end-users are expected to agree to that one.

Most contracts without a specified end date or condition have some variation of that term, to allow for discontinuation of services/products, pricing changes and other issues. These terms often have some guarantee of advance notice before a change is made, but not always. Generally companies believe that their business partners/ suppliers/customers will act ethically; when your counterparty does act unethically, a contract is of little solace anyway.

A typical example of this type of contract would be a shipping contract, which usually does not guarantee pricing. Another example would be electronic components, which are well-specified, but whose mask (implementation on silicon) can be changed at any time (and notice may or may not be given).