As a reader of Hitler's favourite English paper AOG you will know all about double standards so I would not be so bold as to try to answer your question.
You are comparing a permanent street decoration with a temporary protest. They are not similar at all. I'm pretty sure that the cafe would also have been told to remove the "art" if it had featured the Trump.
Isn’t that obvious? Society is made up of millions of people. Or on a macro level - thousands...isn’t it natural that double ‘ triple ‘ quadruple standards will exist within society?
Let’s leave those aside, because you’ve brought up something genuinely interesting that I want to understand. You wrote:
//...society has expectations and it makes demands. The quest for diversity, the furore over Trump, the penchant for defending Islam – object to any of those things at your peril.//
What does that have to do with societal double standards?
That second half of that statement looks like a non-sequitur.
"And the quest for diversity is de rigueur because society as a whole champions it. "
But does it? I would argue society as a whole couldn't give monkeys. It is the minority right-on liberal elite who have infiltrated important parts of our society such as Police, Judiciary and the media who champion it.
//And the quest for diversity is de rigueur because society as a whole champions it.//
And now you’re qualifying society ‘as a whole’.
But that’s not quite right, because there is a rich vein of disagreement about the benefits of diversity and there are plenty of commentators (again, on the right politically) who VERY are vocal about it.