Yes very wierd! I've come across that reaction to spearmint before, and it is a quite different type of flavour than the 'chillified' stuff really. But its' just one of the Thai taste aversions. Another one they usually can't hack is strong cheese, like blue and stilton.
Please don't try this at home though. Thai people traditionally haven't been used to dairy products and it can make them ill. Milk has been encouraged more in recent years (by His Majesty the King) with flavourings used to make it more palletable. But cheese is still an acquired taste for a few, with the strong stuff usually ending up spat out.
Equally strangely condensed and evaporated milks are used in bucket loads.
I can only put the taste thing down to sugar, which is almost always used in thai cooking to balance out the chilli. Check out the ingredients on your Wrigleys pack to see if this theory stands up - (I can't - don't like the stuff either.)

A sprout is for life - not just for Christmas.