I understand the importance of putting your brand in front of as many people as possible, but sometimes companies get carried away:

I am in favor of adding a meta tag, sure - it's hidden and if someone wants to see what a company is using then by all means do a little digging. I fully support what my company does by adding a tiny branded link at the very bottom of each page, "Powered by Cularis" (in 6px type). It's subtle and can be wiped out with a simple CSS change. But to force feed a graphical button, with a large branded logo on every store is...well, it's not nice. Customers that go through the hours of research to choose an eCommerce platform should feel that they are getting a "custom" solution, one tailored for them but upgraded and maintained by the provider they've selected. If a customer is spending more than $50 a month they shouldn't have to deal with any branding other than their own.

In Closing...

There are many reasons for why Volusion and other companies do this, but it should be an option, and if a customer wants it (read: doesn't mind the advertisment) they should get a discount off the product price. I might be wrong or out of line but I think you will agree.