Flashy UI and solid backend code are almost always done by different people. The designer may be writing markup or some JS, but they won't be hacking on end to end encryption. The alternative then, would be to fire the designer and hire another backend guy.
Except they're not going to just not have any UI design at all, and hiring contractors for a whole project would probably end up being more expensive than a full time person anyway. Other members of the team could do it (probably not as well as a specialist), but then they would actually be spending time on design that they would have spent on the back end, which was your original criticism.
Also, the "flashiness" of the interface has a lot more to do with the talent of the designer than anything else. A talented designer may charge more, but probably not that much more, especially on a feel-good open source kickstarter project.
Except they're not going to just not have any UI design at all, and hiring contractors for a whole project would probably end up being more expensive than a full time person anyway. Other members of the team could do it (probably not as well as a specialist), but then they would actually be spending time on design that they would have spent on the back end, which was your original criticism.
Also, the "flashiness" of the interface has a lot more to do with the talent of the designer than anything else. A talented designer may charge more, but probably not that much more, especially on a feel-good open source kickstarter project.