Your courtesy is not my courtesy. That, to reiterate, is why we have laws.It may not be illegal but it may be discourteous.
The difference is that one is optional and the other not. I am not obliged to hold the door open for you and there is no legal penalty for my not doing so. You are obliged not to make me fearful and if you do so you may find yourself in court and end up with a criminal record.
To answer the inevitable question: if you could prove (beyond a reasonable doubt) that my taking a picture of you had made you fearful and that I did or should have known such would be the consequence of my action then I too could end up in court.
EDIT: I just noticed that @jamesev wrote it already and much more concisely.