It is very surprising that always when something like this happens, a battalion of Thai haters start accusing that the Thais do this and the Thais do that! So, does that justify that ALL foreigners should follow suit?
In reality, we have to accept the fact that the Thais are in general clean - of course, there's always the exception to the rule, not only regarding Thais but any nationality.
Another fact that we have to accept is, that the Indians aren't the most cleanest people on the planet - again, there are exceptions to the rule. This, I have first hand experience on my visits to India and whilst having some Indian colleagues at work.
If anyone is seen littering, it is the responsibility of anyone around to object to that, IMHO.