Well that could sound logical, but we all know that you are not allowed to do anything that will get you free something
Kidding. Well, partly that's true. Even if the contract was clearly noted on the case what you can and cannot do with it, you could argue if the contract is obligatory if you didnt sign it. This could go on and on. So, if you dont want to make contracts, and hire lawyers for buying a cd, better just accept that you will have to presume that modifying it is forbidden.