Hi there! these verbs could be a little difficult at first, however you can easily handle them. Lets see... It says "Each one IS free to enter or leave when they want" we use "ser" for "is" because it actually is something he she or it IS at that very moment, and remains for the determined period of time, if we use "Estar" instead, it would say "Each one stays free to enter or leave when they want" "Estar would then mean you are staying in the position where you can leave or stay. It applies like that in this escenario, my recommendation.... don´t try to remember the rules of when which word should be use (No, Im not crazy, I just teach different) try lo listen or read stuff where they use "ser" and "estar", you will familiarize with it and you will just know it. Why? because it will sounds right to you. Be honest, this sounds crazy for you? trust me it works!

Una persona ES libre de hacer lo que quiera, de tomar sus decisiones. Una persona ESTÁ libre para hacer algo, es decir, tiene tiempo libre para hacerlo: ¿Estás libre esta tarde para ir a tomar un café conmigo?