I am really surprised to hear that this happened to the OP. While I do not travel regularly to Thailand, I do live and work in Europe (Bulgaria now, but previsouly Slovakia). My travel often results results in me being in country for more than 90 days. Staying in these countries more than 90 days out of 180 days requires a visa or a residency permit (both of which I have now). Not once has Delta questioned whether I had the proper credentials to be in Bulgaria or Slovakia for more than 90 days. Only once did an agent ask me if I needed to have a visa to go to Bulgaria, but that was more about the agent not realizing that Bulgaria is part of the EU.