Well, the members of the OCs are named (at least the president, probably all of them) by the IOC and act as its deputies locally, and the IOC by its silence chose to condone the decision made by the Greek local committee. The ethical rules of the organization are there and are technically the same for everyone, it was more a matter of the Greek OC choosing to enforce the rules strictly rather than overlooking the whole thing (as I agree many countries would have done), which as you point out they could hardly do without harm in the current climate.

It's sad for the girl, but she only has herself to blame. She knew or should have known the values and ethics that as an athlete wishing to be part of the Olympics she's supposed to adhere to. No one forced her to use Twitter.