Slackware-live's README contains the following text:
I know that's how sudo works, but I thought "su" would ask for the root user's password.

My apologies if I'm being ignorant by bringing this up. I haven't downloaded the ISO, so I don't know how "su" is configured to behave. But I think that "su" wanting the root password (or the password of the new username) is the traditional behavior.

If "su" is re-configured (or sym-linked to "sudo") for simplification, I can understand. But then, why is the live CD behaving differently from a "real" installation?

The passwords for both accounts are the same....

The root user's password is "root" and the live user's password is "live".

I don't know anything about 'Live', but probably Eric has set suauth OWNPASS option for that user. man 5 suauth for details.

It's kind of a hidden feature that many folk are unaware of, which isn't surprising given it doesn't even get a mention on the su manpage.

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Live and learn. I hope your learnt from the answers to your post.

Indeed I have. And I knew that might be the case when I submitted the OP.