It's been a while since I have tried Zoosk but according to the review
here under the costs section you can only send messages for free but not read replies. So you are correct.
It is disappointing when dating sites don't state up front what is free and what is not on their service as it is a little misleading. I guess they want to get you to try the service with the hopes if you get and messages and replies you will be more motivated to subscribe.
"Free dating sites" are not really free either though. You may be able to send and receive messages but they will hide other important features behind a pay wall making it difficult to use their service without upgrading.