[uticus]

As someone who suffers from this allergy, I'll chime in with 2 additional thoughts:

It's not just that UI development is more ambiguous, it is the fact that it is difficult to make text-based code fit it. This is doubly (or maybe logarithmicaly) so when responsive (same code fit different screens). HTML, XAML, HAML, etc all require a lot of code. Reusability mechanisms (like components, etc) help this but it is a world of difference from what BE devs are comfortable with.

Thought #2: I'd guess BE devs like to just code, and the FE frameworks have a reputation of getting in the way of that. This is a less certain thought, so welcome opposition, but seems at least like part of the emotional reaction a BE dev would have to FE work.