Seek-out and contact the owners of these businesses. Point out what you have discovered. There's an outside possibility that they have no clue that their web designers have done this. They might actually feel rather embarrassed to learn so. If I couldn't code my own sites and had to rely on designers who took this approach I certainly would feel pretty bad upon learning about it.
What to do then? Tell them that you'll gladly allow them to use the design for a small fee. Or, perhaps better yet, if they actually hired an outside firm to build their site, propose that you take over their site design and that you'll fix what they got wrong.
If the deed was done in-house or you are up against a coder-designer-founder that just go lazy, figure out a win-win. Out of respect they ought to at least pay you something. You could even lobby for a link in the footer with "Site design based on ...". I wouldn't opt for "Site designed by" because if they screw it up it could look bad for you.
Aikido vs. Karate. It can work wonders.
Oh, yes, I also concur with those who proposed that you might want to consider productizing your design. As a minimum-viable-product you now know that there are people willing to steal it. With the right approach you might be able to find people willing to buy it.
You could even consider expanding upon this and creating a few more designs. Post them openly on your site with an invitation to use them and the condition that you are to receive payment after a thirty day trial period. Just a thought.