I think the issue has a number of facets:

1) Public land or not? Is the pony club within its rights controlling access? (I suspect yes.)

2) Competing sales. Is Outlaw selling his photos? (He Indicated he did in one post, which appeared to rile Kiwi, although it appears not for this event.)

3) Behaviour of the shouting woman. Why does she think she has the right to tell Outlaw what to do? On face value, her behaviour was not appropriate.

4) Position of the Pony Club. Did they make an exclusive arrangement? Had they conveyed to members and parents that photography was restricted in some way?

I have dealt with pony clubs in the past (as a parent of participants - not a photographer) and the memories make me shudder - in my experience, pony clubs have a substantial proportion of petty rule-driven martinets, who gravitate to positions of power. I recently had to meet someone at the local pony club for completely non-horse reasons, and it still made me twitch to go there .