Ethereum foundation is not the custodian of the network. This is a debatable point, since it's not perfectly clear what that would mean. Ethereum Foundation doesn't own the miners or the validating nodes on the network, and can't control who joins or what software they run. Like other development teams, they can propose and try to drum up support for upgrades, which they've been successful at so far.
Detractors of Ethereum would prefer to view the Foundation as having much greater influence over the network, since that would mean it's less centralized. Proponents of Ethereum on the other hand would say Ethereum Foundation's role is similar to that of Bitcoin Core, except the divisiveness hasn't set in yet (or in the case of Ethereum Classic, opposing factions split off before it got bad).
The Ethereum Foundation owns and controls the trademarks of Ethereum. While they cannot dictate the code the various participants on the network run, they get to explicitly decide which version of the code gets to be called "Ethereum". That's how they maintain control.
There's a good chance that the trademark is no longer enforceable due to their failure to apply enforce it against Ethereum Classic. There has been quite a bit of discussion about this within the community.
Who should they have sued to enforce their trademark in the case of Ethereum Classic? Unless the argument is for genericization as opposed to failure to enforce, I don't understand.
Detractors of Ethereum would prefer to view the Foundation as having much greater influence over the network, since that would mean it's less centralized. Proponents of Ethereum on the other hand would say Ethereum Foundation's role is similar to that of Bitcoin Core, except the divisiveness hasn't set in yet (or in the case of Ethereum Classic, opposing factions split off before it got bad).