There was a British poker player who successfully argued that poker is a game of skill and therefore should not be subject to gambling tax. Can’t remember his name but he won the WSOP in 1982, I think.
I would guess that similar arguments are being made here.
The mahjong buddies that I play with always tell me that it’s OK as long as there aren’t more than two tables going at a certain location. Don’t know if this is really official or just urban legend.
I’ve also joined mahjong tournaments where a location had more than a dozen tables going. However, there was a set entrance fee, a set number of rounds, and set cash prize amounts, so I suppose that’s not considered gambling but more like a sweepstakes? There’s also atawmj.org.tw that regularly advertises these kind of tournaments.
I’d still be curious to see any official laws or regulations.