noticing solecisms in the grammar

DSCN1812, originally uploaded by extemporanea.

This sign has recently popped up at an intersection near our house. It fascinates me not because it's completely futile (the red-shirted gent in the background of the photo is, in fact, one of the cohort of usual traders, whose activities have diminished not one whit since the sign went up) but because its grammatical construction genuinely perplexes me. Why the continuous tense? If that is, in fact, the continuous tense and not some kind of misshapen gerund. The sense seems to be something along the lines of "[This sign is] prohibiting trading in the intersection", which is self-referential and redundant. I find myself mentally filling in chirply little slogans: "This sign! prohibiting trading in the intersection since 2010!"

As usual, it all seems curiously unnecessary - why the hell they couldn't simply say "Trading prohibited at intersection" I do not know, unless it has something to do with wanting the two halves of the sentence to balance out in terms of number of letters, for sheer artistic purposes. Honestly, I sometimes think that sign-painters are raised deliberately in sealed environments into which no vestige of grammatical sense is permitted to intrude.

Interesting sidenote: when I completely ignored the avocado-proffering trader in order to haul out my camera and snap the sign, he immediately wanted to know if I was "some kind of journalist." Only, I fear, the most limited and eccentric kind.