There was no central entity mandating that everyone was required to run the "new" network. It was entirely up to miners and people running nodes to decide which network they supported. Further the decision to back out the DAO hack was done via vote so that's another important part of decentralization called consensus.
Is the choice to run a less valuable network really a choice?
Remember that the foundation people gave themselves most of the coins that will ever exist before the network was even started, even if a lot was sold long before there was a word for ICOs. Only ~ 20% or so of the supply will ever be mined. It is not economically rational to differ in consensus rules from the foundation.
Well yes it is, although people take Ethereum quite seriously the code-base led Vitalik has quite a distance to go before it completes the roadmap delivered some years ago. So it is fair for the foundation to patch hacks as well as provide other upgrades.
It you want to continue to use the hackable version because you believe that code should never change and be immutable and never change then that is also a choice.