You're missing the entire point of buying a bootstrap theme.
>SaSS and Foundations' media responsive layout enable me to create new themes easily and painlessly
Yes, anybody can do that with Bootstrap as well. People who buy themes don't even want to waste that time on customising it. I already have 2 clients who bought a theme on WrapBootstrap because they wanted to have a UI-in-a-box for an internal tool.
>SaSS and Foundations' media responsive layout enable me to create new themes easily and painlessly
Yes, anybody can do that with Bootstrap as well. People who buy themes don't even want to waste that time on customising it. I already have 2 clients who bought a theme on WrapBootstrap because they wanted to have a UI-in-a-box for an internal tool.