Guys, chill!
It's basically a case of local and global social norms.
When an argument does erupt, there really is no law that allows public seats to be reserved. This is the same message we're trying to teach our young in the school libraries, that you can't "chope" or reserve a seat in the school library using your books.
Typically, expats who have been briefed before hand will try to "understand" this peculiar social norm that he/she won't find in any part of the world, except in cafeterias of prisons/high-schools (where gang/bully activities reign), and in, well, Singapore.
But of course there will always be a p*ssed off expat caught on a very bad day, who needless to say has been sick and tired of this public property reservation scheme. That's when things get ugly. Sadly, both are correct in different ways. The expat is correct to claim the empty seat as this is a PUBLIC seat where no law exists about reserving them. The person who reserved it with a tissue/bag/newspaper/you-name-it is also, ermmm, kinda correct, as it is a social NORM that he/she has been surrounded with. It's a case of NURTURE, good/bad upbringing, or whatever you call it.
So I just hope everyone will play nice, and try to understand. We'll have a better world to live on (ooohhh, that just sounds so cheesy, haha!)