To all Thai people this is just something that they have always grown up with. For me when I first went to a Bangkok theater nearly 20 years ago it was a bit of a surprise to me: In the middle of the previews and advertisements that take place before every movie a homage to the King of Thailand comes on and a message in English appears next to the Thai script.
I don't remember what the actual statement is but it is something along the lines of "Please stand up and pay respect to His Majesty, the King of Thailand."
I don't mind doing this. The song itself is quite lovely and it is only about 90 seconds long. A bunch of heart-warming images of the current King and the much-beloved, recently passed King come up on the screen. I have been in cinemas in the past where a group of foreigners did not stand up right away and theater ushers came and made them do so. There is a very good reason for this and those foreigners should consider themselves lucky that the ushers did come and "make them" stand up.
Although not standing at a theater is a minor offense and it is very unlikely that any sort of punishment would be doled out to a group of unknowing foreigners who may not have even been looking at the screen when the request came up, this country holds the Royal Family in the absolute highest regard. Lese majeste is no joke here and while a royal pardon or reduction in sentence is likely to occur, these cases are very open-and-shut. Basically, you do NOT want to find yourself as a defendant in such a case. Even if you are pardoned you will without question be permanently banned for ever returning to Thailand.
I think there are a lot of differences between Thais and westerners and our notions of the royal families that exist in Europe are considerably less devotional than that of the Thai people. When Thai people hear about people publicly defacing images of the English Royal Family it is something unfathomable. I find the public's unfaltering love for the Royal Family endearing and after all, it really isn't interfering with your life very much to have to put down your popcorn for 90 seconds and watch a nice montage of pictures accompanied by a rather lovely song.
Although this warning isn't really necessary for a vast majority of people who would probably be respectful anyway, not to mention the fact that you WILL be reprimanded by theater staff for not complying, just do it. The best-case scenario an offender will face is being kicked out of the theater without a refund - worst case scenario is significantly worse.