Around 50 million Mainlanders visit Hong Kong every year.
Tourism Commission - Tourism Performance
So around 140000 per day. Even if we have one public shitting incident per day (More like one per month)...well you do the math.
Most cases I've heard about seem to be elderly people, children or people with disabilities...we don't live in a perfect world. If you're new to Hong Kong, you could be mistaken for assuming there would be toilets and I think it's perfectly plausible that within such a huge volume of people there will be one or two who get 'caught short' or don't know the rules.
Get over yourselves for God's sake.
To add, I'm more offended by people who whip out their phones to photograph people pooing...would you do that? If you saw an old lady having a poo would your first thought be to photograph it and upload it to Facebook?
Really?
That's offensive...that's offensive.