In that announcement, they gave notice that a) service could continue if a successor was found, and b) users were still able to revoke the authority you gave them to post on your behalf:

Until a successor is found that wants to continue this service, all operations and services are halted. The login will be kept available, as well as the Lock/Unlock App, so people can safely remove authorization to post on their behalf from their account.

-from Shutdown of Streemian.com

Again, if you didn't change your voting weight or revoke authorization, then you gave tacit consent for this to happen.

Of course it could have been avoided had they rejected the reward. It also could have been avoided if users chose to practice due diligence and educate themselves on a service they chose to use.