This seems to be a thread I missed early on, but perhaps a comment is still worthwhile(?).
With frets on a fiddle, it really isn't necessary to finger "behind the fret" as one would on most other instruments. For most fretted instruments, you pluck once and there is no further input to the string, so it's desirable to have the string undamped, to "ring" with as much sustain as possible. Increasing the "sustain" or "after ring" quite probably was a major reason for adding frets to the banjo.
While "beginner" or otherwise less accomplished players might have appreciated the easier "finding the spot" with the frets, the real reason for frets on the banjo was to MAKE IT LOUDER (As if it needed it.) - i.e. to make each note last a bit longer and get rid of the "plink-plink" characteristic sound of the (primitive) unfretted banjo.
More modern banjos, fretted or unfretted have been pushed toward improving the sustain and "tone quality," so the difference between fretted and unfretted probably is less apparent than when frets first began to be used a lot.
With a fiddle, you have a continuous and sustained "power input" to the string from the bow, so the damping that comes from holding the string down with a "squishy finger" really doesn't matter much. Fingering directly on top of a fret should give almost exactly the same result as fingering behind the fret.
While it's not possible to be sure from the pictures linked here, older but similar "clip on fretboards" that I've seen (very rarely) had frets much too low/small to effectively be called frets in the usual sense of the word, and provided only a "center" for the finger on the string. In the very few cases where I've talked to anyone who was using one, they were fingering directly on the "bump miscalled a fret" and were not actually "fretting" the string in any usual sense.
Whether it's a helpful learning tool, or an impediment to learning, was - and presumedly is still - much debated. I've only seen it used by beginners, none of whom to my knowledged progressed very far, but then a lot of fiddle beginners don't get very far regardless of the learning/teaching methods used.