Each block chain fork is in essence a difference currency, and so you can tell which transactions happened for a given currency. Which currency is the favored one depends on the client hence the user. The users then just need to make sure that they are selecting the currency that they want to select.

The checkpoint hashes are not imposed on the users, but rather the users must choose to upgrade to the new checkpoint hashes. As such, you can argue that it is decentralized.