To be fair the good MS people did not mention that your synced data will ever be deleted, they only mention that you can disable the sync (by removing the account) and surely once you have removed the account you won't be able to access anymore the already synced data, a good question would be what happens if you re-add the account, will earlier synced data be accessible again?

Regardless whether the data is deleted or not, I find troubling that this synchronization with the Microsoft cloud is forced on the user (if I get this right you cannot add an e-mail account to outlook without opting in for this synchronization).