The bootstrap markup is actually more semantic than Semantic UI. Classes like large, red, floated, right, circular etc.. are presentational not semantic. If you want to use a framework and keep semantic CSS then look in to using the LESS/SASS version of Bootstrap or Foundation.
Edit: After a more detailed look, the framework is actually really nice (and very comprehensive). The name still doesn't make much sense though.
I don't understand this. It's CSS. Of course it's presentational. Do CSS class names have to describe the semantic structure, as well as the DOM structure?
No, it is about marking up the data (HTML) with labels that have a representation described in the CSS.
So the HTML should say "this is important" (em) while the CSS is free to define "important" as bold, different color or some other font change.
For example the HTML should not contain anything about number of columns. It will be 1 column on iPhone, 2 on iPad and 4 on Desktop - but the HTML does not change.
Edit: After a more detailed look, the framework is actually really nice (and very comprehensive). The name still doesn't make much sense though.