The spokesman said MasterCard last notified member banks in October 2007 about a 0.2-percentage-point increase in fees effective from January 2008 under cross-border agreements. Since then, no other fee increases have been made related to ATM charges.
If memory serves, there's already an interchange fee, which many U.S. and other banks pass on to customers for withdrawing from any ATM that is not the bank's. Some, like Wells Feego, also have an invented fee just for using an ATM outside the U.S.
Given that, I'll bet 100% of the 150 baht is unnecessary. I don't think any costs have suddenly increased that'd justify a surcharge, and the area is a declining-cost industry anyway.
I was last there in February, but it seems like Siam Commercial and Bangkok Bank are the real villains and K-Bank, UOB and Ayudhya the good guys; not sure where I'd place TMB.