Mint 17.3
This looks like a deliberate decision in Linux Mint. I just freshly installed Mint 17.3 on a VM, and the root account has a password set in /etc/shadow. After changing my user password, su - accepts my previous user password.
I can't (yet) explain why though.
Mint 18.3
I've just done a fresh install of Mint 18.3, and I don't have a password set for my root account. sudo grep root /etc/shadow shows ! in the password field, which means that the account is locked.