Tom Uren and grugq did a podcast on this recently in Risky Business News [0] I think the main point was that these attacks are physical, don't scale very well, and are untargeted because they depend on luck, so an adversary probably would use more efficient techniques. Unless of course these USB charging points happen to magically cluster around important secret carrying government buildings or other points of interest.

[0] https://risky.biz/BTN46/