Is it true that if I buy XRP they can be taken from me without my consent? not by anyone but by the owners of Ripple
meaning that it is not decentralized and you don't really own any XRP.. you only share them with the owners and trust that they will not take them back?
No, that is not true. XRP is a crypto currency, and you need an account's secret crypto key to do anything with the XRP held by that account. The validator network validates the cryptographic signatures on transactions and only accepts those that are properly signed with the correct keys.
I wouldn't say you own XRP's if someone else can freeze them..