For public use, I'd Ask the Webmaster as a courtesy -- regardless of
legality.  After all, it is (presumably) their content and they should have
some say in whether it appears on another site.  If my content were used to
drive traffic at another site, I'd probably want to know who and where,
etc.

There is an implicit connection between a data feed and the site hosting
it.  You probably don't want your "New Books" feed featured at a porn site,
even though it might really increase your circ.  :)

I always like to think of these kinds of things as "win-win" so my attitude
would be ...  I love your content but I see it but you don't have an RSS
feed.  Would you mind if I kind of wrote one?  Would you like to use that
code to help other people?

And hey, while we're chatting, here's some information about RSS feeds!  Or,
here's the name of my consultant friend who helps people setup RSS feeds for
a reasonable price ...

Okay, maybe you won't do that but I am always hawking my consultant friends,
so there you go.