As the previous poster said, this is a UA thing, not a Thai thing. UA doesn't want to spend the money it costs to put its frequent flyers on other airlines' metal. To accomplish this, it blocks Mileage Plus members from redeeming on many, many *A flights that are available to all other *A members.
This issue was the straw that broke the camel's back for me. I'm done with United and have comped over to CO.