Why phone encryption without screen lock is possible under Marshmallow?

Since phone encryption is irreversible, I'd like to ask _before_ applying it to my phone for the first time. I have a slide-screen unlock set under my Moto E3 / Android 6.0 Marshmallow. No PIN, no pattern. How it is possible that my phone / Android allows me to encrypt my phone in this situation? Here are all the warnings I can see. First screen warns me that: - I need to power up my battery to something around maximum, - I need to connect my phone to charger, - phone encryption takes 1+ hour. Second screen warns me that: - phone encryption is irreversible, - phone encryption takes 1+ hour. But, on both screens I can tap Encrypt Phone button without any problems. What am I missing? Shouldn't I be rather warned that phone encryption is not possible in my case, because I failed to set PIN-lock or pattern-lock for my screen / phone? Shouldn't Android 6.0 rather disallow me to continue with phone encryption in this case?