I think the general rule here is that you can remove the Powered by Zencart, but if it says Powered by, it has to be Zencart.

Makes sense, since it is, indeed, Zencart that's powering your site, and it seems to me to be a small enough way to say thanks for a great piece of software.

There are a few of us who won't review a site without that in the footer, too.