Not just hotels and not just farang.
They will rip off anybody who will fall for it!!
I have learned from experience when on some occasions even my Thai wife, who normally stands no nonsense, I have been the one to say, I know what is going on here and I tell them so!
One example, last year at Mo Chit bus station (after a long bus ride) we needed to get to Don Muang airport. I know it is quite close and doesn't justify a B400 asking rate taxi ride - obviously no meter and him being very rude that I will wait for a proper taxi to appear, that I also know will happen very soon. After less than two minutes, such a thing arrives and takes us to the airport on the meter and I think it cost B95
Tourists and the inexperienced will always be vulnerable in sometimes difficult local circumstances - maybe raining or in a hurry!