Then frankly you must be a bit dim they offer the service to the site not personally to each individual transaction
ANY private company can set any legal terms they wish, it is the users responsibility to ensure that they comply with those terms, and if the user breaches those terms tough !
same as you can refuse to accept a particular method of payment if you wish to, or if selling something you can chose who you sell to.
It's called freedom of choice and if you chose to use it, then by doing so, you agree to abide by their terms.