While it's ok to criticize the Thai government and even Thai culture openly, it's considered a grave insult to Thai nationhood as well as to the monarchy not to stand when you hear the national or royal anthems.
Radio and TV station in Thailand broadcast the national anthem daily at 8 am and 6 pm in towns and villages (even in some Bangkok nerighborhoods) this can be heard over public loudspeakers in the streets. The Thais stop whatever they're doing to stand during the anthem (except in expected to do likewise. The royal anthem is played just before films are shown is pubic cinemas; again, the audience always stands until it's over.