When I renew one license, all licenses renew?

I’ve developed a process with my client’s to purchase a theme license for each product. In some cases its the same theme license for several projects.

What I find weird is when I renew support for one license, it renews the theme support for ALL licenses?

If legally, I’m supposed to buy 1 license per project and renew each license every year, how am I supposed to do so when they renew all of my licenses everytime I renew support on one license?