Actually, although most car rental agreements also are written where a lot can be argued, car rental is generally a "one and done" contract that does not go on for an extended time period. Other agreements, like insurance policies, credit card agreements, etc., are of an on-going nature and are often modified by one-party as time goes by. There are usually provisions tucked away in those agreements that allow that.
Honestly, are you saying you've never received the little thin sheets in the mail telling you they are modifying terms of your credit card, or provisions in your insurance policy, etc.? Those are sent all the time -- those contracts are not frozen in time to the original agreement date.
In any event, make your own decisions - I grow tired of legal talk, and want to read about scuba.