I dont deny your claims that dual pricing exists here as it does in many places, especially where services are government subsidised.
There are also individually opportunistic instances, like market sellers (where negotiation is expected) or taxis.
That said, name one place in non-rural Bangkok or any other major centre where this is common practice. I eat out regularly both with thai and expat freinds and have not encounter this once in many years.
I am not saying it doesn't happen but it is certainly not the norm.