Maybe the reason is that TG doesn't want to pay LX for F-lounge usage in ZRH
100% Correct.
However LX and other airlines can use F lounge in BKK, why, because Thai chatge LX or the other airline money.
Thai dont want to pay for anything on foreign soil, such as other airline lounge use but they already have the expense in BKK, so get as much extra income as pos..
Great service model !!