This is true.
I many times as a kid contemplated just taking a tray of strawberries from one of those without paying.
But I realized that the only reason they were there was because people actually paid for them and if I took them without paying he might stop and I would ruin it for everybody.
As an adult I love seeking these out for some proper Danish grown berries and I always pay double, just in case a younger version of me comes by and doesn't come to the same realization I did.