Without justifying it, the reason is simple. They are using a front end framework (bootstrap) that many developers use/understand that also supports 99.9% of browsers.

Running a browser without javascript that you still want graphics to display (so not a screenreader or text-based-browser), is part of the .1% they are willing to disappoint.

Do I think it is overkill? Sure. Do I still use jQuery at work even though the vast majority of its once handy features are now baked into JS in the browser by default? Of course.