It is most decidedly a thing with Mainland Chinese - and the closer you get to Mainland China, the worse. Vietnam being an example. Indians are also very bad about this as are Russians. The perception among some is that it is a developing country issue or a racial issue or a lack of education issue, but this misses the real issue, which is societies in which people are encouraged to break rules as a matter of course to gain advantage.
Sure, you will see people blithely unbuckling seatbelts all over the planet - from the US to Thailand to Australia to the UK and beyond. What you will not see is every single person from a single culture proudly flaunting every single rule they can find - from carryons to seatbelts to window shades up to using phones and so on. The Mainland Chinese in particular take pride is showing to others that they were the first to flaunt a rule and they are loathed throughout Asia for their behavior.