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?