That's what happened. Countries had to join. Of course it's normal to expect people to fulfill the obligations for the benefits. But to rely on this to keep the EU together allows for corruption that will become a detriment for everybody.
No need to give all benefits. It just shouldn't be necessary to join.
The EU is in a position to demand this but it's breeding resentments if countries are forced to join. There is no need to give all benefits, just enough that membership is not obligatory.