The very nature of the internet / social media facilitates this sort of sharing without checking/crediting the originator of the content. This is something many photographers (and other creatives) fail to grasp, especially when somebody has simply shared something on a social media site rather than using it commercially for financial gain.
More importantly however... it would seem the photographer himself uploaded his images onto social media and thus wavered his rights (to a degree) as to how they could be used on there (yes, I'm referring to what many photographers wrongly see as a copyright grab). The photographer also makes the assumption that Harry Stiles 'stole' the image direct from his site and was aware of his identity, you've only got to look at how images are shared/doctored on social media to realise there's good chance this may actually be the case.
Rightly or wrongly, he did go in quite aggressively in response to his image posted and the response in this day/age on social media was to be expected. A little bit of thought rather than a simple hot-headed response could have paid dividends if he'd have accepted the 'theft' as the norm and played it to his advantage.